The patch titled epoll: remove unnecessary xchg has been added to the -mm tree. Its filename is epoll-remove-unnecessary-xchg.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: epoll: remove unnecessary xchg From: Tony Battersby <tonyb@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> Acked-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/eventpoll.c | 22 ++++++++-------------- 1 file changed, 8 insertions(+), 14 deletions(-) diff -puN fs/eventpoll.c~epoll-remove-unnecessary-xchg fs/eventpoll.c --- a/fs/eventpoll.c~epoll-remove-unnecessary-xchg +++ a/fs/eventpoll.c @@ -394,27 +394,21 @@ 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); } } _ Patches currently in -mm which might be from tonyb@xxxxxxxxxxxxxxx are origin.patch linux-next.patch vfs-improve-comment-describing-fget_light.patch epoll-dont-use-current-in-irq-context.patch epoll-remember-the-event-if-epoll_wait-returns-efault.patch epoll-remove-unnecessary-xchg.patch epoll-clean-up-ep_modify.patch epoll-use-real-type-instead-of-void.patch make-shm_get_stat-more-robust.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html