details: http://hg.nginx.org/nginx/rev/74edc0ccf27a branches: changeset: 5970:74edc0ccf27a user: Roman Arutyunyan <a...@nginx.com> date: Wed Feb 04 16:22:43 2015 +0300 description: Core: fixed a race resulting in extra sem_post()'s.
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(). diffstat: src/core/ngx_shmtx.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-) diffs (20 lines): diff -r 3281de8142f5 -r 74edc0ccf27a src/core/ngx_shmtx.c --- a/src/core/ngx_shmtx.c Mon Feb 02 21:28:09 2015 +0300 +++ b/src/core/ngx_shmtx.c Wed Feb 04 16:22:43 2015 +0300 @@ -101,6 +101,7 @@ ngx_shmtx_lock(ngx_shmtx_t *mtx) (void) ngx_atomic_fetch_add(mtx->wait, 1); if (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid)) { + (void) ngx_atomic_fetch_add(mtx->wait, -1); return; } @@ -174,7 +175,7 @@ ngx_shmtx_wakeup(ngx_shmtx_t *mtx) wait = *mtx->wait; - if (wait == 0) { + if ((ngx_atomic_int_t) wait <= 0) { return; } _______________________________________________ nginx-devel mailing list nginx-devel@nginx.org http://mailman.nginx.org/mailman/listinfo/nginx-devel