139 lines
4.3 KiB
PHP
139 lines
4.3 KiB
PHP
; High-level synchronization primitives.
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; Mutex: stands for MUTual EXclusion.
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; Allows to enforce that only one thread executes some code at a time.
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; mutex_lock acquires the given mutex, mutex_unlock releases it;
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; if thread 1 holds the mutex and thread 2 calls mutex_lock,
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; thread 2 is blocked until thread 1 calls mutex_unlock.
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; Several threads can wait for the same mutex; when the owner
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; releases the mutex, one of waiting threads grabs the released mutex,
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; but it is unspecified which one.
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; If there is no contention, i.e. no one calls mutex_lock
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; while somebody is holding the mutex, then
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; mutex_lock and mutex_unlock use just a few instructions.
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; This is the fast path.
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; Otherwise, mutex_lock and mutex_unlock require a syscall
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; to enter waiting state and wake someone up correspondingly.
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; Implementation. We use one dword for status and
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; kernel handle for underlying futex to be able to sleep/wake.
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; Bit 31, the highest bit of status dword,
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; is set if someone holds the mutex and clear otherwise.
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; Bits 0-30 form the number of threads waiting in mutex_lock.
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; All modifications of status dword should be atomic.
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struct MUTEX
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status dd ?
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handle dd ?
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ends
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; Initialization. Set status dword to zero and
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; open the underlying futex.
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; in: ecx -> MUTEX
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proc mutex_init
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mov [ecx+MUTEX.status], 0
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push ebx
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mov eax, 77
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xor ebx, ebx
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call FS_SYSCALL_PTR
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pop ebx
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mov [ecx+MUTEX.handle], eax
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ret
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endp
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; Finalization. Close the underlying futex.
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; in: ecx = MUTEX handle
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proc mutex_destroy
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push ebx
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mov eax, 77
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mov ebx, 1
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call FS_SYSCALL_PTR
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pop ebx
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ret
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endp
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; Acquire the mutex.
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macro mutex_lock mutex
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{
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local .done
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; Atomically set the locked status bit and get the previous value.
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lock bts [mutex+MUTEX.status], 31
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; Fast path: the mutex was not locked. If so, we are done.
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jnc .done
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if ~(mutex eq ecx)
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mov ecx, mutex
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end if
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call mutex_lock_slow_path
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.done:
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}
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; Acquire the mutex, slow path.
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; Someone holds the mutex... or has held it a moment ago.
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; in: ecx -> MUTEX
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proc mutex_lock_slow_path
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; Atomically increment number of waiters.
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lock inc [ecx+MUTEX.status]
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; When the mutex owner will release the mutex and wake us up,
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; another thread can sneak in and grab the mutex before us.
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; So, the following actions are potentially repeated in a loop.
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.wait_loop:
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mov edx, [ecx+MUTEX.status]
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; The owner could have unlocked the mutex in parallel with us.
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; If so, don't sleep: nobody would wake us up.
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test edx, edx
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jns .skip_wait
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; Pass the fetched value to the kernel along with futex handle.
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; If the owner unlocks the mutex while we are here,
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; the kernel will detect mismatch and exit without sleeping.
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; Otherwise, the owner will wake us up explicitly.
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push ebx ecx esi
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mov eax, 77
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mov ebx, 2
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mov ecx, [ecx+MUTEX.handle]
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xor esi, esi
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call FS_SYSCALL_PTR
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pop esi ecx ebx
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.skip_wait:
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; We have woken up.
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; Or we didn't even sleep because status dword has been changed beneath us.
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; Anyway, something may have changed, re-evaluate the situation.
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; Atomically set the locked status bit and get the previous value.
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lock bts [ecx+MUTEX.status], 31
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; If the mutex was locked, someone has grabbed the mutex before us.
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; Repeat the loop.
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jc .wait_loop
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; The mutex was unlocked and we have just managed to lock it.
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; Our status has changed from a waiter to the owner.
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; Decrease number of waiters and exit.
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lock dec [ecx+MUTEX.status]
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ret
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endp
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; Release the mutex.
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macro mutex_unlock mutex
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{
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local .done
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; Atomically clear the locked status bit and check whether someone is waiting.
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lock and [mutex+MUTEX.status], 0x7FFFFFFF
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; Fast path: nobody is waiting.
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jz .done
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mov ecx, [mutex+MUTEX.handle]
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call mutex_unlock_slow_path
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.done:
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}
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; Release the mutex, slow path.
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; Someone is sleeping in the kernel, or preparing for the sleep.
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; Wake one of waiters.
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; in: ecx = MUTEX handle
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proc mutex_unlock_slow_path
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push ebx
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mov eax, 77
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mov ebx, 3
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mov edx, 1
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call FS_SYSCALL_PTR
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pop ebx
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ret
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endp
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