Messages in this thread |  | | Date | Tue, 04 Feb 2020 11:41:42 +0100 | From | Roman Penyaev <> | Subject | Re: [PATCH 1/3] epoll: fix possible lost wakeup on epoll_ctl() path |
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Hi Andrew,
Could you please suggest me, do I need to include Reported-by tag, or reference to the bug is enough?
-- Roman
On 2020-02-03 21:59, Roman Penyaev wrote: > This fixes possible lost wakeup introduced by the a218cc491420. > Originally modifications to ep->wq were serialized by ep->wq.lock, > but in the a218cc491420 new rw lock was introduced in order to > relax fd event path, i.e. callers of ep_poll_callback() function. > > After the change ep_modify and ep_insert (both are called on > epoll_ctl() path) were switched to ep->lock, but ep_poll > (epoll_wait) was using ep->wq.lock on wqueue list modification. > > The bug doesn't lead to any wqueue list corruptions, because wake up > path and list modifications were serialized by ep->wq.lock > internally, but actual waitqueue_active() check prior wake_up() > call can be reordered with modification of ep ready list, thus > wake up can be lost. > > Current patch replaces ep->wq.lock with the ep->lock for wqueue > modifications, thus wake up path always observes activeness of > the wqueue correcty. > > Fixes: a218cc491420 ("epoll: use rwlock in order to reduce > ep_poll_callback() contention") > References: https://bugzilla.kernel.org/show_bug.cgi?id=205933 > Signed-off-by: Roman Penyaev <rpenyaev@suse.de> > Cc: Max Neunhoeffer <max@arangodb.com> > Cc: Jakub Kicinski <kuba@kernel.org> > Cc: Christopher Kohlhoff <chris.kohlhoff@clearpool.io> > Cc: Davidlohr Bueso <dbueso@suse.de> > Cc: Jason Baron <jbaron@akamai.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > --- > fs/eventpoll.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/fs/eventpoll.c b/fs/eventpoll.c > index b041b66002db..eee3c92a9ebf 100644 > --- a/fs/eventpoll.c > +++ b/fs/eventpoll.c > @@ -1854,9 +1854,9 @@ static int ep_poll(struct eventpoll *ep, struct > epoll_event __user *events, > waiter = true; > init_waitqueue_entry(&wait, current); > > - spin_lock_irq(&ep->wq.lock); > + write_lock_irq(&ep->lock); > __add_wait_queue_exclusive(&ep->wq, &wait); > - spin_unlock_irq(&ep->wq.lock); > + write_unlock_irq(&ep->lock); > } > > for (;;) { > @@ -1904,9 +1904,9 @@ static int ep_poll(struct eventpoll *ep, struct > epoll_event __user *events, > goto fetch_events; > > if (waiter) { > - spin_lock_irq(&ep->wq.lock); > + write_lock_irq(&ep->lock); > __remove_wait_queue(&ep->wq, &wait); > - spin_unlock_irq(&ep->wq.lock); > + write_unlock_irq(&ep->lock); > } > > return res;
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