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

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Date:	Tue, 13 Sep 2011 16:22:48 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Josh Boyer <jwboyer@...hat.com>
Cc:	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,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] epoll: Fix spurious lockdep warnings

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...

Thanks,

-Jason
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