linux-kernel - Re: pipe/cred lockdep warning

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Date:	Thu, 3 Oct 2013 19:56:30 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	ebiederm@...ssion.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: pipe/cred lockdep warning

On Tue, Oct 01, 2013 at 10:57:55AM -0400, Dave Jones wrote:
> I seem to recall hitting this quite a while ago. Does it look familiar ?
> Either it didn't get fixed, or it's back..

> [ 2836.628506] trinity-child0/30759 is trying to acquire lock:
> [ 2836.628544]  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff81239045>] proc_pid_attr_write+0xf5/0x140
> [ 2836.628624] 
> but task is already holding lock:
> [ 2836.628664]  (&pipe->mutex/1){+.+.+.}, at: [<ffffffff811ccf66>] pipe_lock+0x26/0x30
> [ 2836.628734] 

splice to /proc/<pid>/attr/<something> [cred_guard shite under pipe->mutex]

vs.

splice from XFS [pipe->mutex under exclusive iolock]

vs.

read from XFS in execve() [shared iolock under cred_guard]

Note, BTW, that splice to /proc/<pid>/attr/<something> is broken.
proc_pid_attr_write() is *not* supposed to allow partial writes at all.
Frankly, I'd consider adding a ->splice_write() instance that would
simply return -EINVAL there...
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