epoll: remove unnecessary xchg
xchg in ep_unregister_pollwait() is unnecessary because it is protected by
either epmutex or ep->mtx (the same protection as ep_remove()).
If xchg was necessary, it would be insufficient to protect against
problems: if multiple concurrent calls to ep_unregister_pollwait() were
possible then a second caller that returns without doing anything because
nwait == 0 could return before the waitqueues are removed by the first
caller, which looks like it could lead to problematic races with
ep_poll_callback().
So remove xchg and add comments about the locking.
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index c806a0c..64c5503 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -394,27 +394,21 @@
}
/*
- * This function unregister poll callbacks from the associated file descriptor.
- * Since this must be called without holding "ep->lock" the atomic exchange trick
- * will protect us from multiple unregister.
+ * This function unregisters poll callbacks from the associated file
+ * descriptor. Must be called with "mtx" held (or "epmutex" if called from
+ * ep_free).
*/
static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
{
- int nwait;
struct list_head *lsthead = &epi->pwqlist;
struct eppoll_entry *pwq;
- /* This is called without locks, so we need the atomic exchange */
- nwait = xchg(&epi->nwait, 0);
+ while (!list_empty(lsthead)) {
+ pwq = list_first_entry(lsthead, struct eppoll_entry, llink);
- if (nwait) {
- while (!list_empty(lsthead)) {
- pwq = list_first_entry(lsthead, struct eppoll_entry, llink);
-
- list_del_init(&pwq->llink);
- remove_wait_queue(pwq->whead, &pwq->wait);
- kmem_cache_free(pwq_cache, pwq);
- }
+ list_del(&pwq->llink);
+ remove_wait_queue(pwq->whead, &pwq->wait);
+ kmem_cache_free(pwq_cache, pwq);
}
}