linux-kernel - Re: [PATCH] epoll: Fix spurious lockdep warnings

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Date:	Tue, 13 Sep 2011 14:16:03 -0700
From:	Andrew Morton <akpm@...gle.com>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Josh Boyer <jwboyer@...hat.com>,
	Nelson Elhage <nelhage@...hage.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Davide Libenzi <davidel@...ilserver.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Paul Bolle <pebolle@...cali.nl>, stable@...nel.org
Subject: Re: [PATCH] epoll: Fix spurious lockdep warnings

On Tue, 13 Sep 2011 16:22:48 -0400
Jason Baron <jbaron@...hat.com> wrote:

> On Wed, Sep 07, 2011 at 08:04:29PM -0400, Josh Boyer wrote:
> > On Tue, Aug 09, 2011 at 02:11:55PM -0400, Nelson Elhage wrote:
> > > epoll can acquire recursively acquire ep->mtx on multiple "struct
> > > eventpoll"s at once in the case where one epoll fd is monitoring
> > > another epoll fd. This is perfectly OK, since we're careful about the
> > > lock ordering, but it causes spurious lockdep warnings. Annotate the
> > > recursion using mutex_lock_nested, and add a comment explaining the
> > > nesting rules for good measure.
> > > 
> > > Recent versions of systemd are triggering this, and it can also be
> > > demonstrated with the following trivial test program:
> > > 
> > > --------------------8<--------------------
> > > 
> > > int main(void) {
> > >    int e1, e2;
> > >    struct epoll_event evt = {
> > >        .events = EPOLLIN
> > >    };
> > > 
> > >    e1 = epoll_create1(0);
> > >    e2 = epoll_create1(0);
> > >    epoll_ctl(e1, EPOLL_CTL_ADD, e2, &evt);
> > >    return 0;
> > > }
> > > --------------------8<--------------------
> > > 
> > > Cc: stable@...nel.org
> > > Reported-by: Paul Bolle <pebolle@...cali.nl>
> > > Tested-by: Paul Bolle <pebolle@...cali.nl>
> > > Signed-off-by: Nelson Elhage <nelhage@...hage.com>
> > 
> > Any progress on this heading upstream?
> > 
> 
> Patch looks good to me, feel free to add:
> 
> Acked-by: Jason Baron <jbaron@...hat.com>
> 
> However, I am going to have to re-base the epoll path I recently posted:
> https://lkml.org/lkml/2011/9/2/295, if this goes in first. Perhaps,
> Andrew (added to the 'cc), can help us sort out the ordering...

I have already fixed up epoll-limit-paths.patch.  You're planning on
sending a new version of that patch.  Please do base that on
epoll-fix-spurious-lockdep-warnings.patch.
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