Messages in this thread Patch in this message | | | Date | Mon, 02 Mar 2009 18:10:23 -0500 | From | Tony Battersby <> | Subject | [PATCH 3/5] epoll: remove unnecessary xchg |
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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 wasn't unnecessary, 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> --- --- a/fs/eventpoll.c 2009-02-23 13:26:53.000000000 -0500 +++ b/fs/eventpoll.c 2009-02-23 13:29:31.000000000 -0500 @@ -413,27 +413,21 @@ static void ep_poll_safewake(wait_queue_ } /* - * 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); } }
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