On Fri, 6 Nov 2020 18:16:27 -0500 Soheil Hassas Yeganeh <soheil.kdev@xxxxxxxxx> wrote: > From: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx> > > This patch series is a follow up based on the suggestions and feedback by Linus: > https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjDA@xxxxxxxxxxxxxx Al Viro has been playing in here as well - see the below, from linux-next. I think I'll leave it to you folks to sort this out, please. commit 57804b1cc4616780c72a2d0930d1bd0d5bd3ed4c Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx> AuthorDate: Mon Aug 31 13:41:30 2020 -0400 Commit: Al Viro <viro@xxxxxxxxxxxxxxxxxx> CommitDate: Sun Oct 25 20:02:01 2020 -0400 lift locking/unlocking ep->mtx out of ep_{start,done}_scan() get rid of depth/ep_locked arguments there and document the kludge in ep_item_poll() that has lead to ep_locked existence in the first place Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> diff --git a/fs/eventpoll.c b/fs/eventpoll.c index ac996b959e94..f9c567af1f5f 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -554,20 +554,13 @@ static inline void ep_pm_stay_awake_rcu(struct epitem *epi) rcu_read_unlock(); } -static void ep_start_scan(struct eventpoll *ep, - int depth, bool ep_locked, - struct list_head *txlist) -{ - lockdep_assert_irqs_enabled(); - - /* - * We need to lock this because we could be hit by - * eventpoll_release_file() and epoll_ctl(). - */ - - if (!ep_locked) - mutex_lock_nested(&ep->mtx, depth); +/* + * ep->mutex needs to be held because we could be hit by + * eventpoll_release_file() and epoll_ctl(). + */ +static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist) +{ /* * Steal the ready list, and re-init the original one to the * empty list. Also, set ep->ovflist to NULL so that events @@ -576,6 +569,7 @@ static void ep_start_scan(struct eventpoll *ep, * because we want the "sproc" callback to be able to do it * in a lockless way. */ + lockdep_assert_irqs_enabled(); write_lock_irq(&ep->lock); list_splice_init(&ep->rdllist, txlist); WRITE_ONCE(ep->ovflist, NULL); @@ -583,7 +577,6 @@ static void ep_start_scan(struct eventpoll *ep, } static void ep_done_scan(struct eventpoll *ep, - int depth, bool ep_locked, struct list_head *txlist) { struct epitem *epi, *nepi; @@ -624,9 +617,6 @@ static void ep_done_scan(struct eventpoll *ep, list_splice(txlist, &ep->rdllist); __pm_relax(ep->ws); write_unlock_irq(&ep->lock); - - if (!ep_locked) - mutex_unlock(&ep->mtx); } static void epi_rcu_free(struct rcu_head *head) @@ -763,11 +753,16 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt, ep = epi->ffd.file->private_data; poll_wait(epi->ffd.file, &ep->poll_wait, pt); - locked = pt && (pt->_qproc == ep_ptable_queue_proc); - ep_start_scan(ep, depth, locked, &txlist); + // kludge: ep_insert() calls us with ep->mtx already locked + locked = pt && (pt->_qproc == ep_ptable_queue_proc); + if (!locked) + mutex_lock_nested(&ep->mtx, depth); + ep_start_scan(ep, &txlist); res = ep_read_events_proc(ep, &txlist, depth + 1); - ep_done_scan(ep, depth, locked, &txlist); + ep_done_scan(ep, &txlist); + if (!locked) + mutex_unlock(&ep->mtx); return res & epi->event.events; } @@ -809,9 +804,11 @@ static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait) * Proceed to find out if wanted events are really available inside * the ready list. */ - ep_start_scan(ep, 0, false, &txlist); + mutex_lock(&ep->mtx); + ep_start_scan(ep, &txlist); res = ep_read_events_proc(ep, &txlist, 1); - ep_done_scan(ep, 0, false, &txlist); + ep_done_scan(ep, &txlist); + mutex_unlock(&ep->mtx); return res; } @@ -1573,15 +1570,13 @@ static int ep_send_events(struct eventpoll *ep, init_poll_funcptr(&pt, NULL); - ep_start_scan(ep, 0, false, &txlist); + mutex_lock(&ep->mtx); + ep_start_scan(ep, &txlist); /* * We can loop without lock because we are passed a task private list. - * Items cannot vanish during the loop because ep_scan_ready_list() is - * holding "mtx" during this call. + * Items cannot vanish during the loop we are holding ep->mtx. */ - lockdep_assert_held(&ep->mtx); - list_for_each_entry_safe(epi, tmp, &txlist, rdllink) { struct wakeup_source *ws; __poll_t revents; @@ -1609,9 +1604,8 @@ static int ep_send_events(struct eventpoll *ep, /* * If the event mask intersect the caller-requested one, - * deliver the event to userspace. Again, ep_scan_ready_list() - * is holding ep->mtx, so no operations coming from userspace - * can change the item. + * deliver the event to userspace. Again, we are holding ep->mtx, + * so no operations coming from userspace can change the item. */ revents = ep_item_poll(epi, &pt, 1); if (!revents) @@ -1645,7 +1639,8 @@ static int ep_send_events(struct eventpoll *ep, ep_pm_stay_awake(epi); } } - ep_done_scan(ep, 0, false, &txlist); + ep_done_scan(ep, &txlist); + mutex_unlock(&ep->mtx); return res; }
