Skip to content

Commit

Permalink
unix,win: fix threadpool race condition
Browse files Browse the repository at this point in the history
90891b4 introduced a race
condition when accessing `slow_io_work_running` – it is being
increased and later decreased as part of the worker thread loop,
but was accessed with different mutexes during these operations.

This fixes the race condition by making sure both accesses
are protected through the global `mutex` of `threadpool.c`.

This fixes a number of flaky Node.js tests.

Refs: libuv#1845
Refs: nodejs/reliability#18
Refs: nodejs/node#23089
Refs: nodejs/node#23067
Refs: nodejs/node#23066
Refs: nodejs/node#23219
  • Loading branch information
addaleax committed Oct 4, 2018
1 parent 6781db5 commit 28139bf
Showing 1 changed file with 12 additions and 6 deletions.
18 changes: 12 additions & 6 deletions src/threadpool.c
Original file line number Diff line number Diff line change
Expand Up @@ -62,10 +62,10 @@ static void worker(void* arg) {
uv_sem_post((uv_sem_t*) arg);
arg = NULL;

uv_mutex_lock(&mutex);
for (;;) {
uv_mutex_lock(&mutex);
/* `mutex` should always be locked at this point. */

wait_for_work:
/* Keep waiting while either no work is present or only slow I/O
and we're at the threshold for that. */
while (QUEUE_EMPTY(&wq) ||
Expand Down Expand Up @@ -93,13 +93,13 @@ static void worker(void* arg) {
other work in the queue is done. */
if (slow_io_work_running >= slow_work_thread_threshold()) {
QUEUE_INSERT_TAIL(&wq, q);
goto wait_for_work;
continue;
}

/* If we encountered a request to run slow I/O work but there is none
to run, that means it's cancelled => Start over. */
if (QUEUE_EMPTY(&slow_io_pending_wq))
goto wait_for_work;
continue;

is_slow_work = 1;
slow_io_work_running++;
Expand All @@ -122,13 +122,19 @@ static void worker(void* arg) {
w->work(w);

uv_mutex_lock(&w->loop->wq_mutex);
if (is_slow_work)
slow_io_work_running--;
w->work = NULL; /* Signal uv_cancel() that the work req is done
executing. */
QUEUE_INSERT_TAIL(&w->loop->wq, &w->wq);
uv_async_send(&w->loop->wq_async);
uv_mutex_unlock(&w->loop->wq_mutex);

/* Lock `mutex` since that is expected at the start of the next
* iteration. */
uv_mutex_lock(&mutex);
if (is_slow_work) {
/* `slow_io_work_running` is protected by `mutex`. */
slow_io_work_running--;
}
}
}

Expand Down

0 comments on commit 28139bf

Please sign in to comment.