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 20:31:34 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Jason Baron <jbaron@...mai.com>
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 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@...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...

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