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Core: fixed a race resulting in extra sem_post()'s.

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The mtx->wait counter was not decremented if we were able to obtain the lock
right after incrementing it.  This resulted in unneeded sem_post() calls,
eventually leading to EOVERFLOW errors being logged, "sem_post() failed
while wake shmtx (75: Value too large for defined data type)".

To close the race, mtx->wait is now decremented if we obtain the lock right
after incrementing it in ngx_shmtx_lock().  The result can become -1 if a
concurrent ngx_shmtx_unlock() decrements mtx->wait before the added code does.
However, that only leads to one extra iteration in the next call of
ngx_shmtx_lock().
This commit is contained in:
Roman Arutyunyan 2015-02-04 16:22:43 +03:00
parent cae1b5d63a
commit 2bae4205ab
1 changed files with 2 additions and 1 deletions
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3
src/core/ngx_shmtx.c
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@ -101,6 +101,7 @@ ngx_shmtx_lock(ngx_shmtx_t *mtx)
(void) ngx_atomic_fetch_add(mtx->wait, 1); (void) ngx_atomic_fetch_add(mtx->wait, 1);
if (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid)) { if (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid)) {
(void) ngx_atomic_fetch_add(mtx->wait, -1);
return; return;
} }
@ -174,7 +175,7 @@ ngx_shmtx_wakeup(ngx_shmtx_t *mtx)
wait = *mtx->wait; wait = *mtx->wait;
if (wait == 0) { if ((ngx_atomic_int_t) wait <= 0) {
return; return;
} }