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-rw-r--r--fs/eventpoll.c42
1 files changed, 26 insertions, 16 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index e767e4389cb1..adbe328b957c 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -600,8 +600,13 @@ static void ep_remove_wait_queue(struct eppoll_entry *pwq)
wait_queue_head_t *whead;
rcu_read_lock();
- /* If it is cleared by POLLFREE, it should be rcu-safe */
- whead = rcu_dereference(pwq->whead);
+ /*
+ * If it is cleared by POLLFREE, it should be rcu-safe.
+ * If we read NULL we need a barrier paired with
+ * smp_store_release() in ep_poll_callback(), otherwise
+ * we rely on whead->lock.
+ */
+ whead = smp_load_acquire(&pwq->whead);
if (whead)
remove_wait_queue(whead, &pwq->wait);
rcu_read_unlock();
@@ -1134,17 +1139,6 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
struct eventpoll *ep = epi->ep;
int ewake = 0;
- if ((unsigned long)key & POLLFREE) {
- ep_pwq_from_wait(wait)->whead = NULL;
- /*
- * whead = NULL above can race with ep_remove_wait_queue()
- * which can do another remove_wait_queue() after us, so we
- * can't use __remove_wait_queue(). whead->lock is held by
- * the caller.
- */
- list_del_init(&wait->entry);
- }
-
spin_lock_irqsave(&ep->lock, flags);
ep_set_busy_poll_napi_id(epi);
@@ -1228,10 +1222,26 @@ out_unlock:
if (pwake)
ep_poll_safewake(&ep->poll_wait);
- if (epi->event.events & EPOLLEXCLUSIVE)
- return ewake;
+ if (!(epi->event.events & EPOLLEXCLUSIVE))
+ ewake = 1;
+
+ if ((unsigned long)key & POLLFREE) {
+ /*
+ * If we race with ep_remove_wait_queue() it can miss
+ * ->whead = NULL and do another remove_wait_queue() after
+ * us, so we can't use __remove_wait_queue().
+ */
+ list_del_init(&wait->entry);
+ /*
+ * ->whead != NULL protects us from the race with ep_free()
+ * or ep_remove(), ep_remove_wait_queue() takes whead->lock
+ * held by the caller. Once we nullify it, nothing protects
+ * ep/epi or even wait.
+ */
+ smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);
+ }
- return 1;
+ return ewake;
}
/*