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Why can't you sleep while holding spinlock?
As far as I know, spinlocks should be used in short duration, and are only choices in code such as interrupt handler where sleeping (preemption) is not allowed.
However, I do not know why there is such a "rule" that there SHOULD BE no sleeping at all while holding a spinlock. I know that it is not a recommended practice (since it is detrimental in performance), but I see no reason why sleeps SHOULD NOT be allowed in spinlocks.
You cannot hold a spin lock while you acquire a semaphore, because you might have to sleep while waiting for the semaphore, and you cannot sleep while holding a spin lock (from "Linux Kernel Development" by Robert Love).
The only reason I can see is for portability reasons, because in uniprocessors, spinlocks are implemented as disabling interrupts, and by disabling interrupts, sleeping is of course not allowed (but sleeping will not break code in SMP systems).
But I am wondering if my reasoning is correct or if there are any other reasons.
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It cannot sleep; in fact, it cannot relinquish the processor for any reason except to service interrupts (and sometimes not even then). Any deadlock like mentioned above may result in an unrecoverable state.
The interrupt handler, before accessing the device, must also obtain the lock. 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.
Spinlock is a lock which causes a thread trying to acquire it to simply wait in the loop and repeatedly check for its availability. In contrast, a mutex is a program object that is created so that multiple processes can take turns sharing the same resource. Thus, this is the main difference between spinlock and mutex.
This policy is in place because a spin lock cannot be hold while a semaphore is acquired. If this happens, it would take more than six to seven hours before semaphore is acquired and with spin lock in hold, one cannot do so. Linux is an operating system kernel, and it is like Unix; while semaphore is a variable.
There are several reasons why, at least in Linux, sleeping in spinlocks is not allowed:
Use the right tool for the right job - if you need to sleep, semaphores and mutexes are your friends.
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