On Wed, Aug 03, 2022 at 08:15:04PM +0800, Yu Kuai wrote:
wait_cnt = atomic_dec_return(&ws->wait_cnt);
- if (wait_cnt <= 0) {
- int ret;
+ /*
+ * For concurrent callers of this, callers should call this function
+ * again to wakeup a new batch on a different 'ws'.
+ */
+ if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
+ return true;
If wait_cnt is '0', but the waitqueue_active happens to be false due to racing
with add_wait_queue(), this returns true so the caller will retry. The next
atomic_dec will set the current waitstate wait_cnt < 0, which also forces an
early return true. When does the wake up happen, or wait_cnt and wait_index get
updated in that case?
- wake_batch = READ_ONCE(sbq->wake_batch);.
+ if (wait_cnt > 0)
+ return false;
- /*
- * Pairs with the memory barrier in sbitmap_queue_resize() to
- * ensure that we see the batch size update before the wait
- * count is reset.
- */
- smp_mb__before_atomic();
+ wake_batch = READ_ONCE(sbq->wake_batch);
- /*
- * For concurrent callers of this, the one that failed the
- * atomic_cmpxhcg() race should call this function again
- * to wakeup a new batch on a different 'ws'.
- */
- ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
- if (ret == wait_cnt) {
- sbq_index_atomic_inc(&sbq->wake_index);
- wake_up_nr(&ws->wait, wake_batch);
- return false;
- }
+ /*
+ * Wake up first in case that concurrent callers decrease wait_cnt
+ * while waitqueue is empty.
+ */
+ wake_up_nr(&ws->wait, wake_batch);
- return true;
- }
+ /*
+ * Pairs with the memory barrier in sbitmap_queue_resize() to
+ * ensure that we see the batch size update before the wait
+ * count is reset.
+ *
+ * Also pairs with the implicit barrier between decrementing wait_cnt
+ * and checking for waitqueue_active() to make sure waitqueue_active()
+ * sees result of the wakeup if atomic_dec_return() has seen the result
+ * of atomic_set().
+ */
+ smp_mb__before_atomic();
+
+ /*
+ * Increase wake_index before updating wait_cnt, otherwise concurrent
+ * callers can see valid wait_cnt in old waitqueue, which can cause
+ * invalid wakeup on the old waitqueue.
+ */
+ sbq_index_atomic_inc(&sbq->wake_index);
+ atomic_set(&ws->wait_cnt, wake_batch);
return false;
}
--
2.31.1