Date: Wed, 1 Jul 2009 16:48:16 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Neil Horman <nhorman@tuxdriver.com>
Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
       earl_chew@agilent.com, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space
	pipe readers to complete (v4)
Message-ID: <20090701144816.GA26536@redhat.com>
References: <20090622172818.GB14673@hmsreliant.think-freely.org> <20090630173836.GA15612@hmsreliant.think-freely.org> <20090630174634.GD15612@hmsreliant.think-freely.org> <20090701055151.GB26877@redhat.com> <20090701103109.GA29601@hmsreliant.think-freely.org> <20090701122533.GA11411@redhat.com> <20090701141220.GC29601@hmsreliant.think-freely.org>
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On 07/01, Neil Horman wrote:
>
> On Wed, Jul 01, 2009 at 02:25:33PM +0200, Oleg Nesterov wrote:
> > On 07/01, Neil Horman wrote:
> > >
> > > On Wed, Jul 01, 2009 at 07:52:57AM +0200, Oleg Nesterov wrote:
> > > > > This extra count is reclaimed in
> > > > > +		 * wait_for_dump_helpers
> > > > > +		 */
> > > > > +		pipe = file->f_path.dentry->d_inode->i_pipe;
> > > > > +		pipe_lock(pipe);
> > > > > +		pipe->readers++;
> > > > > +		pipe_unlock(pipe);
> > > >
> > > > why should we inc ->readers in advance?
> > > >
> > > Read the comment immediately above it and look at the filp_close path.  We inc
> > > ->readers in advance so as to prevent pipe_inode_info getting freed between the
> > > time we write out the core file and the time we wait on the pipe.
> >
> > Can't understand.
> >
> > call_usermodehelper_stdinpipe() creates 2 files, both share the same
> > inode/pipe_inode_info (->f_path actually).
> >
> > Until we close the file returned by call_usermodehelper_pipe(),
> > pipe_inode_info can't go away.
> >
> > > If the
> > > userspace helper exits in between those points we inode->i_pipe will be null by
> > > the time we get to wait_for_dump_helpers.
> >
> > See above. Can't understand how this can happen.
> >
> Yes, I apologize, in reviewing that code, I don't see how it could happen
> either.  I think that I must have changed to things at once and gotten erroneous
> results in my testing (most likely I did the wait_for_dump_helpers after the
> filp_close and that cause the race).

Yes, this can explain the bug you hit.

Anyway. I appiled my patch and tested it with "!/bin/true" in
proc/sys/core_pattern. It works even if I add
schedule_timeout_interruptible(3*HZ) before binfmt->core_dump()
to make sure core_pattern task exits before we start to wait.

Oleg.

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