linux-kernel - Re: [PATCH v2 2/3] epoll: ep->wq can be woken up unlocked in certain cases

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Date:   Mon, 10 Feb 2020 13:16:26 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Max Neunhoeffer <max@...ngodb.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Christopher Kohlhoff <chris.kohlhoff@...arpool.io>,
        Davidlohr Bueso <dbueso@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] epoll: ep->wq can be woken up unlocked in certain
 cases



On 2/10/20 4:41 AM, Roman Penyaev wrote:
> Now ep->lock is responsible for wqueue serialization, thus if ep->lock
> is taken on write path, wake_up_locked() can be invoked.
> 
> Though, read path is different.  Since concurrent cpus can enter the
> wake up function it needs to be internally serialized, thus wake_up()
> variant is used which implies internal spin lock.
> 
> 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
> ---
>  Nothing interesting in v2:
>      changed the comment a bit
> 
>  fs/eventpoll.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index eee3c92a9ebf..6e218234bd4a 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1173,7 +1173,7 @@ static inline bool chain_epi_lockless(struct epitem *epi)
>   * Another thing worth to mention is that ep_poll_callback() can be called
>   * concurrently for the same @epi from different CPUs if poll table was inited
>   * with several wait queues entries.  Plural wakeup from different CPUs of a
> - * single wait queue is serialized by wq.lock, but the case when multiple wait
> + * single wait queue is serialized by ep->lock, but the case when multiple wait
>   * queues are used should be detected accordingly.  This is detected using
>   * cmpxchg() operation.
>   */
> @@ -1248,6 +1248,12 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
>  				break;
>  			}
>  		}
> +		/*
> +		 * Since here we have the read lock (ep->lock) taken, plural
> +		 * wakeup from different CPUs can occur, thus we call wake_up()
> +		 * variant which implies its own lock on wqueue. All other paths
> +		 * take write lock.
> +		 */
>  		wake_up(&ep->wq);
>  	}
>  	if (waitqueue_active(&ep->poll_wait))
> @@ -1551,7 +1557,7 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
>  
>  		/* Notify waiting tasks that events are available */
>  		if (waitqueue_active(&ep->wq))
> -			wake_up(&ep->wq);
> +			wake_up_locked(&ep->wq);


So I think this will now hit the 'lockdep_assert_held()' in
__wake_up_common()? I agree that its correct, but I think it will
confuse lockdep here...

Thanks,

-Jason