linux-kernel - Re: waitqueue lockdep annotation

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Message-ID: <dd769c95-3eea-93a4-9da4-cb0024461a9b@akamai.com>
Date:   Thu, 30 Nov 2017 16:38:02 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        eter 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 11/30/2017 03:50 PM, Andrew Morton wrote:
> On Thu, 30 Nov 2017 06:20:35 -0800 Christoph Hellwig <hch@....de> wrote:
> 
>> Hi all,
>>
>> this series adds a strategic lockdep_assert_held to __wake_up_common
>> to ensure callers really do hold the wait_queue_head lock when calling
>> the unlocked wake_up variants.  It turns out epoll did not do this
>> for a fairly common path (hit all the time by systemd during bootup),
>> so the second patch fixed this instance as well.
> 
> What are the runtime effects of the epoll bug?
> 

I don't think there is a bug here. The 'wake_up_locked()' calls in epoll
are being protected by the ep->lock, not the wait_queue_head lock. So
arguably the 'annotation' is wrong, but I don't think there is a bug
beyond that.

Thanks,

-Jason