问题
i have passed through this post and i noticed that in Clifford's answer he said that we shouldn't use mutex in an interrupt, i know that in an interrupt we have to avoid too much instructions and delays ext... but am not very clear about the reasons could anyone clarify me for which reason we have to avoid this?
In case that we want establish a synchronous communication between 2 interrupt driven threads what are the other mecahnism to use if using mutex is not allowed?
回答1:
The original question you cite refers to code on an Atmel ATMegaAVR - a simple 8 mit microcontroller. In that context, one can assume that the mutex machanism is part of a simple RTOS.
In such a system, there is a thread context and an interrupt context. Interrupts are invoked by the hardware, while threads are scheduler by the RTOS scheduler. Now when an interrupt occurs, any thread will be immediately pre-empted; the interrupt must run to completion and can only be preempted by a higher priority interrupt (where nested interrupts are supported). All pending interrupts will run to completion before the scheduler can run.
Blocking on a mutex (or indeed any blocking kernel object) is a secheduling event. If you were to make any blocking call in an interrupt, the scheduler will never run. In prectice an RTOS would either ignore the blocking call, raise an exception, or enter a terminal error handler.
Some OS's such as SMX, Velocity or even WinCE have somewhat more complex interrupt architectures and support variety of deferred interrupt handler. Deferred interrupt handlers are run-to-completion scheduled from an interrupt but running outside of the interrupt context; the rules for blocking in such handlers may differ, but you would need to refer to the specific OS documentation. Without deferred interrupt handlers, the usual solution is to have a thread wait on a some blocking object such as a semaphore, and have the interrupt itself do little more that cause the object to unblock (such as giving a semaphore for example).
Multi-processor/core and parallel processing systems are another issue altogether, such systems are way beyond the scope of the question where the original comment was made, and beyond my experience - my comment may not apply in such a system, but there are no doubt additional complexities and considerations in any case
回答2:
A mutex is typically used to ensure that a resource is used by only one user at any given time.
When a thread needs to use a resource it attempts to get the mutex first to ensure the resource is available. If the mutex is not available then the thread typically blocks to wait for the mutex to become available.
While a thread owns the mutex, it prevents other threads from obtaining the mutex and interfering with its use of the resource. Higher priority threads are often the concern here because those are the threads that may preempt the mutex owner.
The RTOS kernel assigns ownership of the mutex to a particular thread and typically only the mutex owner can release the mutex.
Now lets imagine this from an interrupt handler's point of view.
If an interrupt handler attempts to get a mutex that is not available, what should it do? The interrupt handler cannot block like the thread (the kernel is not equipped to push the context of an interrupt handler or switch to a thread from an interrupt handler).
If the interrupt handler obtains the mutex, what higher priority code is there that could interrupt the interrupt handler and attempt to use the mutex? Is the interrupt handler going to release the mutex before completing?
How does the kernel assign ownership of the mutex to an interrupt handler? An interrupt handler is not a thread. If the interrupt handler does not release the mutex then how will the kernel validate that the mutex is being released by the owner?
So maybe you have answers for all those questions. Maybe the you can guarantee that the interrupt handler runs only when the mutex is available or that the interrupt handler will not block on the mutex. Or maybe you're trying to protect the resource access from an even higher priority nested interrupt handler that also wants to use the resource. And maybe your kernel doesn't have any hangup with assigning ownership or restricting who releases the mutex. I guess if you've got all these questions answered then maybe you have a case for using a mutex within an interrupt handler.
But perhaps what you really need is a semaphore instead. One common application of a semaphore is to signal an event. Semaphores are very often used this way within interrupt handlers. The interrupt handler posts or sets the semaphore to signal that an event has occurred. The threads pend on the semaphore to wait for the event condition. (A semaphore doesn't have that ownership restriction that a mutex has.) Event signalling semaphores is one common way to establish synchronous communication between 2 interrupt driven threads.
回答3:
While there might be unusual cases where there's some problem with using a mutex in an interrupt handler, it's quite common practice and there's nothing wrong with it.
It really only makes sense on systems with more than one core. With just a single core (and no hyper-threading), the mutex would never do anything anyway. If the core is running code that acquires a mutex that interrupt code can acquire, interrupts (or the subset of them that matter) are disabled anyway. So with just one core, the mutex would never see any contention.
However, with multiple cores, it's common to use mutexes to protect structures that communicate between interrupt and non-interrupt code. So long as you know what you're doing, and you have to if you're going to write interrupt handlers, there's nothing wrong with it.
How the mutex blocks and unblocks is heavily implementation dependent. It can put the CPU to sleep and be woken by an inter-process interrupt. It can spin the CPU in some CPU-specific way.
Note that a totally unrelated concept that is often confused with this is using user-space mutexes in user-space signal handlers. That's a completely different question.
来源：https://stackoverflow.com/questions/35011322/why-not-to-use-mutex-inside-an-interrupt