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

[Roman Penyaev]

On 2020-02-10 19:16, Jason Baron wrote:
> 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...

Argh! True. And I do not see any neat way to shut up lockdep here
(Calling lock_acquire() manually seems not an option for such minor
thing).

Then this optimization is not needed, patch is cancelled.

Thanks for noting that.

--
Roman