linux-kernel - Re: waitqueue lockdep annotation

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Date:   Fri, 1 Dec 2017 14:00:33 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: waitqueue lockdep annotation



On 12/01/2017 12:11 PM, Christoph Hellwig wrote:
> On Thu, Nov 30, 2017 at 05:18:07PM -0500, Jason Baron wrote:
>> Yes, but for those cases it uses the ep->poll_wait waitqueue not the
>> ep->wq, which is guarded by the ep->wq->lock.
> 
> True.  So it looks like we have one waitqueue in the system that is
> special in providing its own synchronization for waitqueues while
> entirely ignoring the waitqueue code documentation that states that
> waitqueues are internally synchronized.
> 
> We could drop the lockdep annotation, updated the documentation and
> not add any validation of the assumptions, or just make epoll fit the
> scheme used by everyone else.  So either we can drop these patches, or
> I need to fix up more of the epoll code.
> 

You could leave the annotation and do something like:
s/ep->lock/ep->wq->lock. And then that would remove the ep->lock saving
a bit of space.

Thanks,

-Jason