Spinlock in interrupt handler



Spinlock in interrupt handler

and for this reason, the spinlock operations are found. The main difference between trap and interrupt is that trap is triggered by a user program to invoke OS functionality while interrupt is triggered by a hardware device to allow the processor to execute the corresponding interrupt handler routine. I am working on the nozomi (UMTS modem) driver, which is not in the main kernel. e. Starting with Windows 8, a driver can use the IoConnectInterruptEx routine to register a passive-level InterruptService routine (ISR). InterruptLock() tries to acquire the spinlock (a variable shared between the interrupt handler and a thread) while interrupts are disabled. Indicated by the in_interrupt() macro returning true. Before a spinlock can be used it must be initialized by a call to cyg_spinlock_init. When the timer ISR returns, the s/w interrupt is delivered, running the scheduler. but was interrupted by the hardware interrupt and an interrupt handler tries to take a   Locking can not be used everywhere, i. Forwhatit’sworth,thei386andx86 The original implementation of interrupt handling in Linux uses the __do_IRQ() super-handler, which is able to deal with every type of interrupt logic. 19 Jun 2018 spinlock kernel crash #22 . The most common lock in the Linux kernel is the spin lock. If an interrupt handler tried to claim a spinlock owned by a thread, the interrupt handler would loop forever. After the interrupt is handled processor resumes the interrupted process. So basically spinlocks are much simpler, and faster under short-lived contention, so that's why they tend to be used. • Otherwise, it is possible for an interrupt handler to interrupt kernel code while the lock is held and attempt to reacquire the lock. * make ->poll_controller call interrupt handler directly The overhead of the non-threaded IRQ handler is the same for kernel or userspace irqchip, since the handler just writes 1 to the eventfd. Thus the complete system hangs. Interrupt Context Not user context: processing a hardware irq or software irq. As a kernel control path, the interrupt handler is lighter than a process (it has less context and requires less time to set up or tear down). RT-Thread is an open source IoT operating system from China. If threads spin on all CPU cores like this, no CPU core will be available for the former thread to release the spinlock, causing a deadlock [11]. 3. We already know that this routine is called interrupt handler (or how we will call it ISR or Interrupt Service Routine from this part). Spin locks can be used in interrupt handlers, whereas semaphores cannot be used because they  13 Jul 2018 In a modern OS, kernel modules often use spinlocks and interrupt handlers to monopolize a CPU core to execute concurrent code in atomic  Queued spinlocks is a locking mechanism in the Linux kernel which is . The code spins in a tight loop until the lock is acquired. Hi all, this is my first post to the list, so please be kind. Timers and timer interrupts let us do exactly that. 6. CPU by means of an interrupt The interrupt handler recognizes and handles the event, and then is the spinlock It has several variants, but one of the variant VOID NTAPI KeInitializeInterrupt(IN PKINTERRUPT Interrupt, IN PKSERVICE_ROUTINE ServiceRoutine, IN PVOID ServiceContext, IN PKSPIN_LOCK SpinLock, IN ULONG Vector, IN KIRQL Irql, IN KIRQL SynchronizeIrql, IN KINTERRUPT_MODE InterruptMode, IN BOOLEAN ShareVector, IN CHAR ProcessorNumber, IN BOOLEAN FloatingSave) If an interrupt handler were to attempt to acquire a spinlock that the interrupted code already holds, however, then a deadlock would result: the interrupt handler will spin waiting for the "lock holder" to release the lock, but the lock holder is the kernel code that was preempted by the interrupt! Most of the interrupt handling should be implemented within this function, as opposed to the high-priority KP_IntAtIrql [B. Thus The code shows that the handler being installed is a fast handler (SA_INTERRUPT), doesn’t support interrupt sharing (SA_SHIRQ is missing), and doesn’t contribute to systementropy(SA_SAMPLE_RANDOMismissing,too). In these situations, the NMI can execute an interrupt handler that transfers control to a special monitor program. Embedded Linux Conference, Europe spinlock_irq_save used where not necessary. The thread must have I/O privileges — the privileges associated with being able to manipulate hardware I/O ports and affect the processor interrupt enable flag (the x86 processor instructions in, ins, out, outs, cli, and sti). Interrupt Handling in Linux Interview Questions Explain about the interrupt mechanims in linux? What are the APIs that are used to register an interrupt handler? How do you register an interrupt handler on a shared IRQ line? Explain about the flags that are passed to request_irq(). You need to specify the device and have the system provide you the resources, or if it is non-PMP use LogConfig in the INF file to specify the interrupt vector. Linux kernel offers a synchronization technique for every possible need (you think it, they have it !). So far, we've learned to register an interrupt handler but not to write one. This means that there is a fast IRQ flow handler for the GPIOs that gets called in a chain from the parent IRQ handler, most typically the system interrupt controller. 2 Parallel, Hardware-Supported Interrupt Handling Our approach is hardware-based and prevents unwanted distur-bance without masking any interrupts. – Control flow of procedure call, and interrupt handler. . - RT-Thread/rt-thread PCI Message Signaled Interrupts. Concurrency Another question was, can we use semaphore / mutex in interrupt handlers? To find the answer to these  a pointer to the spin lock to thread_switch, where it will be unlocked once the . [2] This function receives the IRQ number, the name of the function, flags, a name for /proc/interrupts and a parameter to pass to the interrupt handler. Currently IRQF_SHARED type interrupt line is allocated, but it is not appropriate, as the interrupt line isn't shared between different devices, instead IRQF_ONESHOT is the proper type. The interrupt handler is removed when the process exits, instead of when the attaching thread exits. At a minimum, most interrupt handlers need to provide acknowledgment to the device that they received the interrupt. The clock interrupt handler saves the rest of the registers, as well as other machine state, such as the state of the floating point registers, in the process PCB. This is the Linux Device Driver Tutorial Part 23 – Spinlock in Linux Kernel Part 1. When properly used, spinlocks offer higher performance than   However there are some circumstances where a spinlock is appropriate, especially when interrupt handlers and threads need to share access to hardware, and  Interrupt handlers execute in the context of the Spinlocks. Kernel Locking. Taking out a spinlock in an interrupt handler is a legitimate thing to do; that is one of the reasons that spinlock operations do not sleep. In this case, you do not need to use read_lock_irq(), but only read_lock() everywhere (since if an interrupt occurs, the irq handler will only try to grab a read lock, which won't deadlock). Most of the interrupt handling should be implemented within this function, as opposed to the high-priority KP_IntAtIrql [B. Using Passive-Level Interrupt Service Routines. The second type is a mutex ( include/linux/mutex. Any idea why this is happening? Is there any way to confirm that OS did see an interrupt before it calls my interrupt handler? Thanks in advance! Additionally, an interrupt can notify the received data packets of a networking device. We also mask and acknowledge the interrupt to PIC using handler function of our IRQ descriptor. Instead, we redirect inter-rupt requests to a coprocessor and handle them in parallel to the normal program execution. I tried Google but everywhere I am just getting basic working of these things. With pin-based interrupts or a single MSI, it is not necessary to disable interrupts (Linux guarantees the same interrupt will not be re-entered). If the thread doesn't do these things, it might SIGSEGV when it calls InterruptLock(). @mousey: double-acquire deadlocks cannot occur with spinlocks, because if a spinlock is shared with an interrupt handler all lockers of the spinlock must use the interrupt-disabling variant of the lock function. what should be in the interrupt handler is an interrupt handler. spinunlock irq enable로 unlock을 해 주면, 인터럽트가 몇번 disable됐는지 상관 않고 enable돼 버리므로 주의한다. This will releases the spinlock and restores the interrupts using the flags argument. need to use this version if the lock is something that an interrupt handler may try to acquire. Only after an interrupt handler has completed execution may an interrupted CPU resume the execution - Not a big problem if interrupt handlers are short - Interrupt handler may access the data protected by the spin-lock - Should the interrupt handler use the lock? - Can it be delayed trying to acquire a spin lock? - What if the lock is already held by the thread it interrupted? In short words, the open_softirq function defined in the kernel/softirq. → d o u b l e - a cq u i r e d ea dl o c k 22 Description: The InterruptUnlock() function releases a critical section by unlocking the specified spinlock, reenabling interrupts. The term "atomic" here is used the strongest sense, meaning that the operations are indivisible and non-interleavable at the hardware level, even by interrupt handlers, and even on an SMP. Interrupt handler is also called the interrupt service routine. This flag is then used as the value for the from_irq parameter of pl011_tx_chars, causing polling to be used in the unsafe case. Kernel Programming. The limitation at present is that the interrupt controller in the Pi 1, 2 and 3 does not support interrupt priority or interrupt masking so until the current IRQ or FIQ has been cleared by the interrupt handler for the specific device then re-enabling IRQ or FIQ would simply trigger another interrupt immediately and cause an interrupt storm. While there has been some published research [1] on interrupt dispatching on older versions of Windows and on 32-bit systems, not much information is available on how this works on modern Windows systems. Flags for request_irq. By changing interrupt type to IRQF_ONESHOT, now irq handler is no longer needed, as clear of interrupt status can be done in threaded interrupt context. In the rest of the article, I'll use the terms "interrupt handler" and "spinlock section. So for some reason the main queue is not free to dispatch other operations. SMP Symmetric Multi-Processor: kernels compiled for multiple-CPU machines. This isn't always possible - it's impossible to sleep in an interrupt handler, for example. The Arduino Uno has 3 timers: Timer0, Timer1 and Timer2. Besides, WFE takes interrupt masking flag into account so it is not as useful here as WFI. Interrupt. When interrupt occurs and is enabled (which means that it is not masked off and the primary interrupt bit in the processor is turned on), then the processor will stop the current running code, save the PC, disable interrupts, and jump to an appropriate interrupt handler. [8 points]: In xv6, the interrupt handler ideintr uses the lock idelock to ensure that it executes atomically. If KP_IntAtDpc returns with a value greater than zero, WD_IntWait returns and the user-mode interrupt handler will be called in the amount of times set in the return value of KP_IntAtDpc. Spinlock debugging. This function will only succeed if there isn't already a handler on this IRQ, or if you're both willing to share. disable_irq not only disables the given interrupt but also waits for a currently executing interrupt handler, if any, to complete. The equipment is informed that the interrupt is handled. Because the interrupt handler can potentially gain control of the machine, we don't let just anybody associate an interrupt. wilcox@hp. As already mentioned, the address of the interrupt handler for IRQ n is initially stored in the interrupt[n] entry and then copied into the interrupt gate included in the proper IDT entry. However, if the driver does too much work in the interrupt handler, system respon-siveness will be degraded. , time quantum expires). ¾If the specified IRQ is shared, the handler identified by the dev_id is unregistered int free_irq( unsigned int irq, void *dev_id) My driver's interrupt handler is called a lot and every time when the handler checks interrupt register of the device, it shows there's no pending interrupt. This is the Series on Linux Device Driver. An interrupt handler is a segment of code responsible for taking the appropriate actions to process a given There is a fairly common case where an interrupt handler needs access to a critical region, but does not need write access. This could become very inefficient. Each device that generates interrupts has an associated interrupt handler. 01 — 4 January 2006 Application note Document information Info Content Keywords Spurious interrupts, LPC2000, IRQ, FIQ, Interrupt Abstract Describes spurious interrupts and how they can be effectively handled in the ARM7 LPC2000 family code executed by an interrupt or by an exception handler is not a process. The aim of this series is to provide easy and practical examples that anyone can understand. If this results in a context switch (or if the interrupt handler refers to data that was being used) then atomicity could be compromised. Linux Kernel Teaching¶. Be aware that if the thread calling disable_irq holds any resources (such as spinlocks) that the interrupt handler needs, the system can deadlock. Semaphores cause tasks to sleep on contention, which is unacceptable for interrupt handlers. 2. Examples. It's like a fruit basket - you just ought to know what you want. The timer will actually call us to let us know it is time to check the clock! Arduino Timers. y git tree can be found at: git://git. This causes the interrupt handler to run. static irqreturn_t *lock, unsigned long flags );. SA_INTERRUPT, SA_SHIRQ, SA_SAMPLE_RANDOM. • The interrupt handler spins, waiting for the lock to become available. *PATCH v4 3/3] irqchip: add a SiFive PLIC driver 2018-08-09 7:55 simplified RISC-V interrupt and clocksource handling v4 Christoph Hellwig 2018-08-09 7:56 ` [PATCH v4 1 So if you need to synchronize code while handling an interrupt, you must use a spinlock. 10. But this never happens, because the _start function only continues to run after the interrupt handler returns. The interrupt handler, before accessing the device, must also obtain the lock. Because the former has a save and restore semantic when disabling interrupts, so that you will restore interrupt status instead of enable it blindly. or else a deadlock situation could occur. function of an interrupt controller? An interrupt is an external signal. No spinlock is not always needed in interrupt handler. 15 gives an overview of interrupt handling. Conversely, if the critical section needs to sleep, you must use a mutex as a thread must not be scheduled, preempted, or put to sleep after it has acquired a spinlock. a spinlock) to prevent this, but may be inefficient. adds the handler at the end of any existing handlers (default is to add in front of existing handlers). I wrote these notes to record what I discovered when I investigated whether it is save to call a timer function like mod_timer while holding a spinlock that is also used by the timer handler. acquire() interruptsare disabled and the spinLock starts to spin, then unless the value of the spinlock is changed to FREE the processor will not be able to do anything. – caf Aug 12 '10 at 4:55 I have a doubt as to what happens when a spinlock is held in process context and at that instant an interrupt occurs ? Can this scenario occur ? As per my understanding if it so happens that the handler too try to acquire the same lock held in process context , it would keep on spinning for that lock. context interrupt handling, replacement of non-preemptible locking with preemptible mutex-based locks . (CORE0) secure-interrupt, dispatch to secure. static IFF you know that the spinlocks are never used in interrupt handlers, you can use  Unlike semaphores, spinlocks may be used in code that cannot sleep, such as interrupt handlers. This variant assumes no interactions with interrupt handlers and this lock. When an interrupt occurs, the interrupt line and the pointer to the device will be passed to the registered Saving registers is the first task of the interrupt handler. Use semaphores in any other situation that calls for code synchronization. May be happen 'DATA-ABORT'. the other interrupt happens on another CPU, but it is _not_ ok if the interrupt happens on the same CPU that already holds the lock I got it from the bellow 2 points: 1). The methods covered will be as follows: • Non-nested interrupt handler • Nested interrupt handler • Re-entrant nested interrupt handler • Prioritized interrupt handler Embedded systems have to handle real world events such as the static irqreturn_t b43_interrupt_handler(int irq, void *dev_id) { • Must use this form whenever a spinlock is used in both interrupt and process context 19. c source for shared interrupt. To guarantee atomicity with respect to the interrupt handlers. Please note one thing first – When an interrupt of a particular device occur on interrupt controller, that interrupt is disabled at interrupt controller and hence on all the cores for that particular device. In Linux, interrupt handlers are normal C functions, which match a specific prototype and thus enables the kernel to pass the handler information in a standard way. The handler should perform the actions that are necessary for the current interrupt. a)It’s sufficient for threads to mask interrupts while accessing X b)In addition, threads must lock (blocking) mutexes before accessing X c)Instead of b, threads must lock spinlocks before accessing X d)In addition to c, the interrupt handler must lock a spinlock before accessing X Unregistering interrupt handler ¾Unregister a specified interrupt handler and disable the given IRQ line if this is the last handler on the line. g. DPC interrupt handler run (and on which it calls thread_switch)? Recall that  Spinlocks and interrupts. In case no interrupt received the IOCTL request that checks for interrupt hangs. The role of the interrupt handler depends entirely on the device and its reasons for issuing the interrupt. It will be woken automatically by any interrupt handler that runs on this CPU, and in an SMP system, you'd have to make to have SEV instructions to wake other CPUs. Outline • spin_lock and semaphore in linux kernel – Introduction and difference. Since ISRs execute in an interrupt context at an elevated IRQL, and since a system might have You can use a spin-lock instead! When a thread tries to acquire a mutex and if it does not succeed, either due to another thread having already acquired it or due to a context switch, the thread goes to sleep until been woken-up which is critical I mean, it probably has some overhead that's avoidable. Call every support routine that uses a spin lock at an appropriate IRQL, generally at <= DISPATCH_LEVEL for executive spin locks or at <= DIRQL for an interrupt spin lock associated with the driver's interrupt objects. It is possible to use any standard locking technique (e. You can call this function from a thread or from an interrupt handler. When it happens, ISR handler stub can safely acknowledge interrupt to the controller and return - if line is still asserted, interrupt will fire again immediately after ISR handler stub acknowledges it to the controller. And your interrupt routine will also have to lock that same spinlock. Then the corresponding interrupt handler is executed. Now the interrupt handler also needs the lock to access the device. Timers + Threads Socket families are implemented through pointers to functions (net_proto_family and proto_ops) Methods to put the nRF52 to sleep in a "spinlock loop" { /* If an interrupt occurrs right at this point, the interrupt handler is executed, the busy flag may be The WRITER is locked, so the interrupt handler waits until it becomes free. 由於 down/up 的 semaphore API 是 sleeping waiting 的版本,因此在 interrupt mode 必須改用 spinlock,spinlock 是以 busy waiting 的方式做 wait operation。 Linux device driver 安裝 interrupt handler 的方式是呼叫 request_irq() 函數,透過此函數來佔用 IRQ,並且安裝interrupt handler。 Request IRQ The interrupt handler, before accessing the device, must also obtain the lock. However, if the PIL of the deivce is above the high priority interrupt level, then initializing the lock with the iblock cookie makes it a spinlock. Interrupt latency is defined as the time interval from an interrupt event to the execution of handler code. Synopsis: #include <sys/neutrino. – An hardware interrupt (IRQ1) is generated when a key is pressed or released – Interrupt handler talks to the hardware and gets the scan code back. The ISR or Interrupt Handler Routine can be found in Interrupt Vector table that is located at fixed address in the memory. • Spinlock: non-blocking spin lock irq(l), spin unlock irq(l). h ): it is like a spinlock, but you . So, how do we avoid such scenarios? Spinlock code is designed in such a way that any time kernel code holds a spinlock, the preemption is disabled on the local processor. The interrupt handler is required to  In software engineering, a spinlock is a lock which causes a thread trying to acquire it to simply The longer a thread holds a lock, the greater the risk that the thread will be interrupted by the OS scheduler while holding the lock. " By interrupt handler, I mean a routine that serves either IO interrupt (installed using install io interrupt) or timer interrupt (scheduled using add timer). If the interrupt handler runs on the same processor, it will start spinning. 2), an ASCII string and a unique device identi er. It's important to release the lock as soon as possible. If this is called from NMI handler when the kernel was interrupted, you do not need neither spinlock nor spinlock_enter() (but I believe that this code is only used for the first case). interrupt Two types of spin lock What if we call spinlock_acquire twice? Why interrupt handler shouldn’t block? Interrupt handler can happen anytime It runs in the context of interrupted process Interrupt == unexpected function call to the interrupt handler So… spinlock_acquire(&lock); … spinlock_release(&lock); Process A The interrupt handler is normally marked static because it is never called directly from another file. * spin_lock_irqsave (spin lock irq save) spin_lock_irq과 같지만, irq disable하기 직전의 interrupt enable To exclude interrupt handlers: mask or disable interrupts, in the hardware; To exclude other processors: use spinlock (busy-wait), using a bus-atomic polling operation; This is not inherently a hierarchy; that is, none of these logical mechanisms completely subsumes the other. By spinlock section, I mean the critical section protected by a spinlock. This common handler saves the hardware state and then calls do_irq (in arch/i386/irq. Deferred work that runs in interrupt context is also known as bottom-half, since its purpose is to execute the rest of the actions from an interrupt handler (top-half). It's designed to be safe to use from user applications, and may be a NOP. Calling KeAcquireSpinLock or KeAcquireInStackQueuedSpinLock sets the IRQL on the current processor to DISPATCH_LEVEL until a corresponding call to KeReleaseSpinLock or KeReleaseInStackQueuedSpinLock restores the previous IRQL. [*] The interrupt handler list can be iterated only by a single thread / context at a time. This is the Linux Device Driver Tutorial Part 24 – Read Write Spinlock in Linux Kernel (Spinlock Part 2). Different from spin lock, thread goes sleep for waiting lock when failed to get the lock. Adaptive locks are the most common type in Solaris and they can either 'block' or 'spin' depending on whether the process/thread holding the lock is running or sleeping respectively whereas, in the case of spinlocks, context switching is not allowed (by raising the interrupt level of the processor when the lock is acquired thereby making sure [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ The vfio interrupt handler (threaded or not) runs at a higher priority > > current handler is that it - Not a big problem if interrupt handlers are short - Interrupt handler may access the data protected by the spin-lock - Should the interrupt handler use the lock? - Can it be delayed trying to acquire a spin lock? - What if the lock is already held by the thread it interrupted? Spin locks can be used in interrupt handlers Semaphores cannot be used because they sleep If a lock is used in an interrupt handler, you must also disable local interrupts Otherwise, it is possible for an interrupt handler to attempt to reacquire the lock (double-acquire deadlock) Currently IRQF_SHARED type interrupt line is allocated, but it is not appropriate, as the interrupt line isn't shared between different devices, instead IRQF_ONESHOT is the proper type. Implementing a Handler. If it is possible, disabling interrupts may be the most efficient method of ensuring atomicity The Interrupt Handler. All users of the 4. For example, disabling interrupts partially subsumes disabling This means that there is a fast IRQ flow handler for the GPIOs that gets called in a chain from the parent IRQ handler, most typically the system interrupt controller. Interrupt handler spin lock not recursive: same thread locking twice will deadlock. Spinlocks just sit in a busy loop until the lock becomes Linux Device Driver Tutorial Part 24 – Read Write Spinlock in Linux Kernel (Spinlock Part 2) This is the Series on Linux Device Driver . c), which in turn (indirectly) calls handle_IRQ_event. If you want to disable interrupts while acquiring a lock, you typically want to use spinlock_irqsave instead of spinlock_irq. Most device drivers have a per-device spinlock which is taken in the interrupt handler. The actions performed in the interrupt handler include reading the device registers to get the status of the device and acknowledge the interrupt, operations that most of the time can be performed with non-blocking calls. Interrupt handling algorithm - Save context - Handle “highest priority” interrupt Wake Interrupt handler thread. The specific case I tracked down consisted of several processes in a tight gettimeofday() loop, which resulted in the reader count never Therefore, the whole ISR chain is going to get invoked until someone returns TRUE. Spinlock. interrupt handler, which is a segment of code responsible for taking the appropriate actions to process the interrupt. Typically, a driver's InterruptService or InterruptMessageService routines (ISRs) must share access to driver data and hardware resources with other driver routines. Do I must control exclusive-access for PL310, secure and non-secure ? (ex. 10/17/2018; 4 minutes to read; In this article. (Why?) 7. – If it is SHIFT/CTRL/ALT, some internal states are changed. Starting with framework version 1. s. Control . (CORE0) In secure, try to execute a write-operation to PL310' register. Basically, for such a short and fast task (interrupt  The choice between a spinlock and another construct which causes the caller to Interrupt handlers can not be rescheduled on Linux, but often do have to use  interrupt handlers run in interrupt context – cannot sleep. The driver code also can not run to release the lock. Before calling this function, the thread must request I/O privileges by calling: ThreadCtl( _NTO_TCTL_IO, 0 ); gpio_driver: add per-pin interrupt handlers 1. Processes, fork and exec The only way processes can be created in a Unix system is through the fork system call. The key problem is that we need consistent lock usage in all places (either all in IRQ-safe or all in IRQ-unsafe), regardless of whether The interrupt handler fills the transmit hardware FIFO until the FIFO is full or the output buffer is empty. Calling Support Routines That Use Spin Locks. A non threaded irq handler cannot have any sleeping calls whether or not you hold any spinlocks. Rather, it is a kernel control path that runs at the expense of the same process that was running when the interrupt occurred. •When a spinlock is used to protect data structures shared with interrupt handlers, the spinlock must disable the execution of such handlers! •In this way, the kernel cannot be interrupted when it holds the spinlock! Since the entire time quantum for the first thread must complete before the next thread can execute and (possibly) release the lock, a spinlock is very wasteful of CPU time, and should be used only in places where a mutex cannot be used, such as in a hardware exception handler or low-level interrupt handler. Spin lock v. Event handler, interrupt handler, signal handler, memory handler, file input handler are some examples for handlers. Although installing the interrupt handler from within the module's initialization function might sound like a good idea, it often isn't, especially if your device does not share interrupts. interrupt handler와 일반 thread가 spinlock을 공유하는 경우에 사용한다. Interrupt handler attempts to acquire the spinlock. c. • The interrupt handlers run in normal kernel threads, so that the priorities of the different interrupt handlers can be configured • The real interrupt handler, as executed by the CPU, is only in charge of masking the interrupt and waking-up the corresponding thread • The idea of threaded interrupts also allows to use sleeping spinlocks InterruptLock() protects access to shared data structures between an interrupt handler and the thread that owns the handler. I mean, if you do this for NMI interrupting userspace, spinlock would work. A second process (or an interrupt handler) that to grab the spinlock will busy wait. I Concurrency 1. InterruptLevel is the IRQ number that ACPI will use. The following factors complicate driver code that handles hardware interrupts on multiprocessor systems: Each time a device interrupts, it provides interrupt-specific information that is volatile because it can be overwritten the next time that the device interrupts. Consider a situation where the interrupt handler Timer_ISR(), as shown in Listing 3, is being serviced, and Task_1, running on another processor, then disables interrupts and enters its critical code section. 1 IRQ-Safe Spin Locks The Linux kernel features several spin lock variants that are safe for using with interrupts. Implement routines to execute as quickly as possible while they hold a spin lock. The code Firing the delayed interrupt only once is not a problem in my case because it's a "ready for more data" signal. 04/20/2017; 4 minutes to read; In this article. Interrupt handlers can not enter sleep, so to avoid calls to some functions which has sleep. Fig. irq_exit() may process softirq or wake softirqd - Iterate over active interrupts (arch dependent) - Schedule - Restore context returning either to previous process or to newly scheduled process Interrupt handler thread executes I'm announcing the release of the 4. • when data is shared with bottom halves, before holding spinlock we must disable bottom halves. The IOCTL handler of the "check for interrupt" is: ***** Here we increment the interrupt count received by this CPU, this is maintained for accounting purpose. So if another CPU gets that IRQ 7 interrupt while you have the spinlock, it will wait for you to release the spinlock before continuing. in_irq() returns true in a hardware interrupt handler. Therefore, its very important to hold a spinlock for minimum possible time. “But the CPU is going to vector off to into hard interrupt context and start executing the handler associated with that network device. Of course, if this code runs on a uniprocessorsystem, it is a disaster. Whereas spinlock just ensures that no other thread will run even if it has to. Tim Jones Published . This is called a spinlock because a thread spins waiting for the lock to  Spinlocks are most commonly used in interrupt handlers where blocking is not an option and, if a lock is held, another CPU will be releasing it in very short order. It is a block of code associated with a specific interrupt condition. SA_SHIRQ installs a shared handler, and the third flag asserts that interrupt timestamps can be used to generate system entropy. Upon receipt of an interrupt, a CPU halts the execution of the task it is currently executing and immediately executes an interrupt handler. 13 Jul 2018 In a modern OS, kernel modules often use spinlocks and interrupt handlers to monopolize a CPU core to execute concurrent code in atomic  //Interrupt handler for IRQ 11. The signal might also come from an interrupt handler, for example if the thread is waiting for the completion of some I/O operation, and an interrupt will stop the execution of the thread temporarily, signal the lock, and resume the thread. 在 interrupt handler 裡,叫醒真正負責此中斷的 task 後立即結束執行。 2. Spinlock contention is very expensive Code holding spinlocks cannot sleep. 31 Oct 2007 Kernel atomics, spinlocks, and mutexes. ACPICA will take care of them. 06/16/2017; 2 minutes to read; In this article. keep the critical  Lesson 1: Spin locks The most basic primitive for locking is spinlock. The output semaphore is signaled to let the upper half know bytes of space are available in the output buffer. If the PIL is below the high priority interrupt level then the lock will be a 'blocking' lock irrespective of whether it is initialized with iblock_cookie or not. • spinlock automatically disables preemption, which avoids deadlock caused by interrupts. Context is the parameter to be past to the Handler. 8] interrupt handler. 2. These code paths, called critical sections, require some combination of concurrency or re-entrancy protection and proper ordering with respect to other events. This function tries to acquire the spinlock (a variable shared between the interrupt handler and a thread) while interrupts are disabled. If this happens  Kernel level synchronization; Spinlocks; Mutexes and system semaphores; Process states; Wait When returning to kernel-space from an interrupt handler. Best Regards, Elliott A Linux device driver beginner 1. The way to implement this is to call request_irq() to get your interrupt handler called when the relevant IRQ is received. Handler is an internal function of ACPICA which handles interrupts. To understand what nested lock is, imagine situation when a thread acquired lock, but was interrupted by the hardware interrupt and an interrupt handler tries to take a lock too. Whenever there is global data accessible by both user code (the user task, that is, the system call) and interrupt code, this shared data should be protected by spin_lock_irqsave() in the user code. Whenever an interrupt occurs (either hardware or software), the CPU stops running the currently executing programs and activities. interrupt handler. spinlock) Please help me. The fix involves adding a per-port flag that is true iff running from within the interrupt handler and the spinlock has not yet been released. but won't the interrupt handler would be able to acquire the lock even if the interrupt occurred in the middle of the critical section --- bad! behaviour of interrupts: spin lock irqsave, spin lock irq, spin lock bh and . org [netdrvr] interrupt handling overhaul * eliminate the work loops in the interrupt handlers. The interrupt handler can be installed either at driver initialization or when the device is first opened. h> void InterruptUnlock( intrspin_t* spinlock); Arguments: spinlock The spinlock (a variable shared between the interrupt handler and a thread) used in a call to InterruptLock() to lock the handler. disable_irq_nosync, on the other hand, returns immediately. com Abstract An interrupt is a signal to a device driver that there is work to be done. » How do you do this . NTO_INTR_FLAGS_PROCESS. In depth explanation of Spinlock, Semaphore, Mutex It can't be acquired by interrupt handler or bottom half A spinlock is quite-literally a tiny, two WFE IMO is mainly for synchronization hints between cores in multicore system (for example doing WFE on spinlock taking fail and issuing SEV after going out from spinlock). mentioned that tcp_cong_control (and thus ccp_invoke) is in a chain of calls stemming from an interrupt handler. • What is Context – What is “context”. Simple char driver with irq Module initialization – Register a major number – Reserve needed GPIOs – Request an IRQ Connect to interrupt service routine – Prepare deferred work 1. The updated 4. If a thread sleeps when holding a spinlock, another thread that requests the same spinlock will spin on a CPU core to wait until the former thread releases the spinlock. threads as well as by an interrupt handler. Kernel Timers. The CPU is never heard from again. 04/20/2017; 6 minutes to read; In this article. And spin locks can not be used within interrupt handlers, because if an interrupt handler tries to acquire a lock that is already held by the process it interrupted, it deadlocks. But what happens if the interrupt routine executes in the same processor as the code that took out the lock originally? While > wasn't for the fact that spinlock debugging checks are still > disabled, during initialization, when iommu_setup() (which then end > up calling set_iommu_interrupt_handler()) is called. This means that the GPIO irqchip handler will be called immediately from the parent irqchip, while holding the IRQs disabled. What if the spinlock is shared between the thread T1 and interrupt handler? In depth explanation of Spinlock, Semaphore, Mutex Seeking in depth explanation for Spinlock, Semaphore, Mutex. Hardware Interrupt / Hardware IRQ Hardware interrupt request. This is a collection of lectures and labs Linux kernel topics. Timer interrupt handler forces current thread to “call” thread_yield. Although SMI interrupts are handled by 1 processor at a time, it even effects real-time responsiveness on dual-core/SMP systems, because if the processor handling the SMI interrupt has locked a mutex or spinlock, which is needed by some other core, that other core has to wait until the SMI interrupt handler has been completed and a mutex Synchronizing Interrupt Code. However, the interrupt disable bit in the CPU would certainly be cleared in the meantime. Full preemption strips local interrupt disabling from spinlock_t operations, but not for raw_spinlock_t operations. It is possible to call these functions from an interrupt handler, but enabling your own IRQ while handling it is not usually good practice. Its handler runs, sees that an alarm has expired, and queues a s/w interrupt to run the scheduler. The interrupt handler (Listing 7) reads the mouse status and checks whether anything has changed since the last interrupt. A handler function (ISR – interrupt service routine) is performed in the context of the disabled hardware interrupts. When you want to share your spinlock with an interrupt you must use irqsave variant. – Dead lock example of spin_lock. no need to worry about interrupts on other CPUs – spin lock will work normally. Also, semaphores cannot be used by interrupt handlers, as Linux doesn't allow interrupt handlers to sleep, so anything that protects interrupts needs to be a spinlock. Hold the spinlock before accessing any element of the IRQ descriptor for our interrupt line. that interrupt handler runs (see Nested Interrupts and High Priority,  Let's look at acquire() in spinlock. interrupt occurs (e. When the interrupt handler has part of the code to enter the critical section, use spinlocks lock, rather than mutexes. Anywhere you use a spinlock to protect a critical section, you should disable interrupts. Release a critical section in an interrupt handler. a) It's sufficient for threads to mask interrupts while accessing X b) In addition, threads must lock (blocking) mutexes before accessing X c) Instead of b, threads must lock spinlocks before accessing X d) In addition to c, the interrupt handler must lock a spinlock before accessing X Spinlocks are meant to be used in interrupt handlers because interrupt execution can’t sleep. We can set up a timer to interrupt us once per millisecond. Also, we reduce modifications to a single thread at a time using a mutex. 5/2. wait_event, mutex, semaphore, etc, are all sleeping calls and must not be used in a non threaded irq handler. Interrupt Handler Migration and Direct Interrupt Scheduling for Rapid Scheduling of Interrupt-driven Tasks JUPYUNG LEE and KYU HO PARK Korea Advanced Institute of Science and Technology In this article, we propose two techniques that aim to minimize the scheduling latency of high- Einige Interrupt-Handler (in Linux als schnelle Interrupt-Handler bekannt) laufen mit allen Interrupts auf dem lokalen processor deaktiviert. again, no need to spin in uniproc – just ++preempt_count & disable irq. “Say you have one thread executing in user mode, and an external interrupt such as a network event fires that you don’t care about in your real time app,” said Cartwright. c source code file and executes initialization of the deferred interrupt handler. that the GPE interrupt handler was being called with local interrupts enabled because acpi_gbl_gpe_lock was implemented as a spinlock_t. – Otherwise the handler converts the scan code into an ASCII character depending on 2. (CONFIG_SMP=y). However, they have different semantic regards interrupt handling. Real time issues - kernel - irqs off examples non-threaded interrupt handler Interrupt handler 便要根據中斷的特性,來判斷此中斷是要請求驅動程式將資料寫入至裝置,或是由裝置讀取資料。 Linux 驅動程式的 interrupt handler 實作原則如下: 1. For example, if processor 0 runs iderw and processor 1 runs the interrupt han-dler ideintr, then the lock idelock ensures that their critical sections are appropriately ordered. Even if the two threads were running at the same priority, the one attempting to claim the spinlock would spin until it was timesliced and a lot of CPU time would be wasted. So if you want to block interrupts on ALL CPU's for a given level, you will have to set a spinlock after setting the TSKPRI register. The interrupt latency (T il) in the above diagram represents the minimum latency—that which occurs when interrupts were fully enabled at the time the interrupt occurred. Theoutbcallthenenablesinter-rupt reporting for the parallel port. The latter finally loops through all the device-driver supplied interrupt handlers And, shall I invoke the tasklet_schedule directly in the IRQ handler or I should schedule a tasklet to run in module_init and implement a queue to consume the interrupt signal between IRQ handler and tasklet? Which is better for the performance wise? Thanks in advance. Spinlocks that are used from interrupt handlers must themselves  communication with peripheral devices; implement interrupt handlers a spinlock primitive, and in (B) we must disable interrupts AND use a spinlock primitive. module_init and implement a queue to consume the interrupt signal between IRQ handler and threads as well as by an interrupt handler. Driver's interrupt handler is called. If you need to write your own interrupt handler, you are not required to use a bottom half. add ISR handler apis so that users of different layers can hook their own isr handler on different GPIO. Microsoft has changed the way interrupts are dispatched in recent versions of Windows. The scheduler acquires its own spinlock, wakes up thread C, which was waiting on the alarm that just expired, and sees that thread C has a higher priority than A. 10 kernel series must upgrade. The control functions are used to set, clear, and get the input and output flags for the UART driver. 8. they are no longer needed, now that NAPI and other asynchronous tasks handle our work (and can be independently controlled by means other than disable_irq or spin_lock_irqsave). If your driver creates several interrupt objects and must synchronize its handling of several interrupts, as might be necessary to support MSI, you can use a single, driver-supplied spin lock for all of the interrupt objects. For interrupts, a program called an interrupt handler is installed, and it is the interrupt handler that handles the interrupt from the disk. Early interrupts handler The spinlock is a low-level synchronization preserve/not preserve previous interrupt state in the flags and The reason you cannot use plain spin_lock() if you race against interrupt handlers is because if you take it and then an interrupt comes in on the same CPU, it will busy wait for the lock forever: the lock holder, having been interrupted, will not continue until the interrupt handler returns. Performance of adding and removing interrupt handlers is not critical. On RT kernels however can you call eventfd_signal from interrupt context? You cannot call spin_lock_irqsave (which can sleep) from a non-threaded interrupt handler, can you? You would need a raw spin lock. (If the mouse is smart enough, it will only interrupt us when something has changed, however let's Line - 581 to 583 Here we increment the interrupt count received by this CPU, this is maintained for accounting purpose. You can however, hold a spinlock to prevent interruption to your interrupt handler. Spinlocks are a different form of locking mechanism, usually used on multiprocessor systems. If you know you are waiting for a volatile flag or spinlock or something, then it's an appropriate mechanism. disable_irq will not only disable the given interrupt, but will also wait for a currently executing interrupt handler, if any, to complete. For this case, each processor has not just copy of the mcs_spinlock structure but array of these structures: This function tries to acquire the spinlock (a variable shared between the interrupt handler and a thread) while interrupts are disabled. Includes a high-level overview. The device interrupts. , the interrupt number of the issuing device), a function pointer to the interrupt handler, interrupt ags (see Section 1. Now if in Lock. This is based on Linux kernel 3. The nub that an interrupt is associated with will be * longer lived than the interrupt; as a result, our owner may not be the first to register for a * particular interrupt index with that nub, so we need to inherit the existing statistics (as we describe * statistics in terms of {nub id, index}, not in terms of our owner). The only peculiarity is that a handler runs at interrupt time and, therefore, suffers some restrictions on what it can do. On an uniprocessor machine the second process will lock the CPU not allowing the first process to wake up and release the spinlock so the second process can continue, it is basically a deadlock. This is the warning that was triggered:-----[ cut here ]----- GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together These calls are used to register and unregister an interrupt handler. The interrupt handler either returns, or it returns and causes an event to be delivered. 8 Nov 2015 interfaces (APIs) to implement interrupt handling in order to meet certain means of mutexes and spinlocks, depending on the execution  8 Aug 2013 Essential information for programming in the OS X kernel. Synchronization with the device interrupt. This is provided by the following primitives, which work on both uniprocessor and SMP machines: InterruptLock(intrspin_t* spinlock) Attempt to acquire a spinlock, a variable shared between the interrupt handler and thread. First, each PCI function is only allowed a single interrupt. It’s really for device drivers that are going to be part of Linux and are expected to have the best possible performance. Unreliable Guide To Locking The fundamental type is the spinlock in_irq() returns true in a hardware interrupt handler. Our solution improves on the one pro- We cant use a mutex in an interrupt because interrupts cant sleep THE MUTEX from CS CS598 at University of Illinois, Urbana Champaign Mutex Spinlock Short Lock If a process grabs a spinlock and goes to sleep before releasing it. request irq() expects as arguments an interrupt line (i. While the lock is held, the device issues an interrupt, which causes your interrupt handler to run. The system call is implemented through an interrupt call (INT 80H. When the associated interrupt occurs, the kernel's interrupt trap handler schedules this routine to run at IRQL Reason: Support your driver take a spinlock that control access to the device and then issues an interrupt. The problem with semaphores is that they put the process to sleep until the semaphore is available. 1 kernel. • when data is shared with interrupt handler, before holding spinlock we must disable interrupts. Line - 588 to 589 However, inside an interrupt handler, you must always use a spinlock because a mutex can put a thread to sleep. The UART interrupt on model B+ is interrupt 81. As Jan mentioned, The implication from disabling interrupts while acquiring a lock is that the lock is also being acquired by some interrupt handler. So the interrupt handler was _never_ able to complete and other interrupts were blocked by that processor (in my case, network and keyboard interrupts). This causes the other thread which send other IOCTL requests to hang also. The flags can include SA_SHIRQ to indicate you're willing to share the IRQ with other interrupt handlers (usually because a number of hardware devices sit on the same IRQ) and SA_INTERRUPT to indicate this is a fast interrupt. SA_INTERRUPT requests installation of a fast handler (as opposed to a slow one). The irq handler does not to use spin_lock_irq() , because the softirq cannot  You can use a spin-lock instead! When a thread tries to acquire a mutex and if it does not succeed, either due to another thread having already acquired it or due   31 Dec 2009 And because you can't sleep in an interrupt handler, these must be spinlocks. Worst-case interrupt latency is this time plus the longest time in which the OS, or The InterruptUnlock() function releases a critical section by unlocking the specified spinlock, reenabling interrupts. For that reason, the device driver must register an interrupt handler function, which  23 Jan 2018 Consider similar situation, where thread1 holds a spinlock and device issues an interrupt to be handled. This depends on the OS, but generally you can't acquire a mutex in an interrupt handler because mutexes block when contended, placing the process on a wait queue, putting the process to sleep, and scheduling a new process to run. M. It goes without saying that you are in an atomic context and must only use spinlock for concurrency. When an interrupt occurs, the hardware automatically switches a part of the context (at least enough to allow the handler to return to the interrupted code). , interrupt handlers can't spin-lock. For this reason, depending on the interrupt type, the flow handlers as modified by the pipeline code may have to mask the interrupt line until the in-band handler has run from the in-band stage, lifting the interrupt cause. So, if I understand correctly, the only safe way for me is the noInterrupts() and wrapping write() with it, because after this line I can be sure that the interrupt won't be fired and it won't be skipped if the pin got high during critical section. • when data is shared with interrupt handler, before holding spinlock  4 Feb 2019 Need spinlocks to block on all CPUs in all cases: semaphore, signal, . You cannot fill in a resource structure and get an interrupt, or any other resource for that matter. I have a issue with the tty code. Otherwise, if a thread holding the spinlock is interrupted by an interrupt handler that tries to acquire the same spinlock, you get a deadlock. Put simply: Use spinlocks to protect critical sections in interrupt-handling code. Usually there is a certain number of IRQs available. Actually, there's nothing unusual about a handler—it's ordinary C code. If the interrupt handler must return one of the following values: 10. In response to a clock interrupt, the OS saves the PC and user stack pointer of the currently executing process, and transfers control to the kernel clock interrupt handler, 2. A maskable interrupt is a hardware interrupt that may be ignored by setting a bit in an interrupt mask register’s (IMR) bit-mask. associates the interrupt handler with the process instead of the attaching thread. When an interrupt has occurred: (Code flow from top to bottom) [GIC: Global Interrupt Controller] For the SoCs which are using ARM GIC The AMD64 architecture allows software to define up to 15 external interrupt-priority classes. Legacy PCI INTx interrupts work, but they have several limitations. And for that latency sensitive code it could matter. Use spinlock for interrupt context and mutexes if you plan to sleep holding the lock Implement a module avoid patching the kernel main tree To defer work implement two halves. But what happens if the interrupt routine executes in the same processor as the code that took out the lock originally? Spinlocks in Interrupt Handlers Use the appropriate API variant They disable interrupts on the calling CPU whilst the lock is held Use to synchronize interrupt handler and non-interrupt code Entering critical region: spin_lock_irqsave(spinlock_t *lock, unsigned long flags) spin_lock_irq(spinlock_t *lock) Exiting critical region: AN10414 Handling of spurious interrupts in the LPC2000 Rev. For this reason, in essentially all multiprocessor systems, the spinlocks that you use for mutual exclusion in interrupt code disable and re-enable interrupts for you. US5560018A - Providing external interrupt serialization compatibility in a multiprocessing environment for software written to run in a uniprocessor environment - Google Patents An interrupt is an asynchronous hardware signal is-sued from a system device to a system CPU. Each device driver registers a set of driver-specific interrupt handlers for all interrupts its associated device may raise. Figure 15 Interrupt handling procedure [33] The jitter in the busy state corresponds to the sum of three factors [33]: 1. If a device uses multiple interrupts, the driver must disable interrupts while the lock is held. Interrupt handler and lock. // For some reason it is allocated to 87 by RASPIAN. Audio project has different software layers, they need different gpio isr handler for layer instead of processing all GPIO interrupts in one handler. Note: if there is no The jump to common_interrupt comes from the actual list of interrupt handler pointers, which start at irq_entries_start. protected by spinlock Interrupt handler Interrupt? 12 Free Electrons. After handling the interrupt, it switches back to the usual tasks it was executing. A better solution would be to use a new exclusion lock available to both the thread and the interrupt handler. When an interrupt occurs, the CPU pauses the task it is currently executing and executes the corresponding interrupt handler, which is known as Interrupt Service Routine (ISR). In our case the deferred function is the run_timer_softirq function that is will be called after a hardware interrupt in the do_IRQ function which defined in the arch/x86/kernel/irq. CR8 uses the four low-order bits for specifying a task priority and the remaining 60 bits are reserved and must be written with zeros. Consider this scenario: Device driver acquires a spinlock. kernel. What this does reveal is that all the spinlock issues have been on long-running queries, for what it is worth. The fundamental issue surrounding locking is the need to provide synchronization in certain code paths in the kernel. fork is one of the many system calls implemented by the linux kernel. 🔗Provoking a Deadlock The data passed to install_io_interrupt_handler() will be passed to the handler function each time it's called. That means that when an IRQ is triggered and the handler is executed, if the handler needs to sleep That is, these are distinguished from objects of the types spinlock_t and rwlock_t. 11, Kernel-Mode Driver Framework (KMDF) and User-Mode Driver Framework (UMDF) drivers running on Windows 8 or later versions of the operating system can create interrupt objects that require passive-level handling. The interrupt handler for a device is part of the device’s driver (the kernel code that manages the device). So now it can generate new interrupts. spin_lock_irq() / spin_unlock_irq() disable & enable irq assuming it was disabled to begin with to designing an interrupt handler and discuss the various trade-offs between the different methods. The lectures focus on theoretical and Linux kernel exploration. The lock holder, however, does not run until the interrupt handler completes. Interrupt handler  spinlock automatically disables preemption, which avoids deadlock caused by interrupts. Here we have the system clock interrupt handler, Timer_ISR(), running concurrently with Task_1(), both accessing the global variable time. Priority classes are numbered from 1 to 15, with priority-class 1 being the lowest and priority-class 15 the highest. Kernel, drivers and embedded Linux development, consulting, training and support // The UART interrupt on RASPI2,3 is interrupt 57 according to the BCM2835 ARM Peripherals manual. I’ll Do It Later: Softirqs, Tasklets, Bottom Halves, Task Queues, Work Queues and Timers Matthew Wilcox Hewlett-Packard Company matthew. Therefore spinlocks are only appropriate for SMP systems where the current owner of a spinlock can continue running on a different processor. It can be anything that might be of use to the handler, or NULL. Timers are another type of deferred work that are used to schedule the execution of future actions after a certain amount of time has passed. to use a spinlock here. Dead Lock Analysis of spin_lock() in Linux Kernel (english) 1. Therefore, you must never hold a spinlock when calling functions outside your file that you are not sure will not sleep. Reason: Support your driver take a spinlock that control access to the device and then issues an interrupt. To make the poll and read functions work we have to consider how to handle the mouse interrupt. Today’s processors are so fast that it may easily be fine to put all the interrupt handling in the basic interrupt handler. Second, PCI INTx interrupts use a separate signal from the address and data lines used for PCI data transactions. Hence, this is the functional difference between Handler and Thread. 8-rc1. Originally, Russell King identified different types of handlers to build a quite universal set for the ARM interrupt handler implementation in Linux 2. Dies wird gemacht, um sicherzustellen, dass der Interrupt-Handler ohne Unterbrechung so schnell wie möglich läuft. 1 Jul 2015 Tag Archives: spinlock. Synchronizing Access to Device Data. You can call this function from a thread or from an interrupt handler. Moreover, handler helps to perform a specific task based on an event while thread helps to increase performance and allows resource sharing. \$\endgroup\$ – Shrimp Apr 22 '12 at 10:33 ACPI_STATUS AcpiOsInstallInterruptHandler(UINT32 InterruptLevel, ACPI_OSD_HANDLER Handler, void *Context) ACPI sometimes fires interrupt. Difference Between Maskable and Non Maskable Interrupt Definition. This pattern really works well in Linux. 3. We must also avoid doing a lot of work in the interrupt handler and instead use deferred work if needed. Supporting Passive-Level Interrupts. • Should the interrupt handler use the lock? • Can it be delayed trying to acquire a   10 Aug 2005 In PREEMPT_RT, normal spinlocks (spinlock_t and rwlock_t) are preemptible, Almost all interrupt handlers run in process context in the  6 Jan 2015 Control flow of procedure call, and interrupt handler. spinlock in interrupt handler

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