linux-kernel - Re: [PATCH 1/3] epoll: fix possible lost wakeup on epoll

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Date:   Tue, 04 Feb 2020 11:41:42 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Max Neunhoeffer <max@...ngodb.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Christopher Kohlhoff <chris.kohlhoff@...arpool.io>,
        Davidlohr Bueso <dbueso@...e.de>,
        Jason Baron <jbaron@...mai.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] epoll: fix possible lost wakeup on epoll_ctl() path

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@...e.de>
> Cc: Max Neunhoeffer <max@...ngodb.com>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Christopher Kohlhoff <chris.kohlhoff@...arpool.io>
> Cc: Davidlohr Bueso <dbueso@...e.de>
> Cc: Jason Baron <jbaron@...mai.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-kernel@...r.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;