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

From: Jason Baron
Date: Mon Feb 10 2020 - 13:18:53 EST




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@xxxxxxx>
> Cc: Max Neunhoeffer <max@xxxxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: Christopher Kohlhoff <chris.kohlhoff@xxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dbueso@xxxxxxx>
> Cc: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> 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