linux-kernel - Re: mmotm 2010-04-05 - another RCU whinge (not network this time)

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Date:	Fri, 09 Apr 2010 23:22:32 -0400
From:	Valdis.Kletnieks@...edu
To:	paulmck@...ux.vnet.ibm.com, Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: mmotm 2010-04-05 - another RCU whinge (not network this time)

On Fri, 09 Apr 2010 16:16:14 PDT, "Paul E. McKenney" said:
> On Thu, Apr 08, 2010 at 07:57:28PM -0400, Valdis.Kletnieks@...edu wrote:
> > On Mon, 05 Apr 2010 16:09:45 PDT, akpm@...ux-foundation.org said:
> > > The mm-of-the-moment snapshot 2010-04-05-16-09 has been uploaded to
> > > 
> > >    http://userweb.kernel.org/~akpm/mmotm/
> > 
> > Hit another one.  I seem to be on a roll...
> > 
> > Seen in dmesg, happened near end of the initrd..
> > 
> > [   26.756864] 
> > [   26.756866] ===================================================
> > [   26.756869] [ INFO: suspicious rcu_dereference_check() usage. ]
> > [   26.756871] ---------------------------------------------------
> > [   26.756874] fs/proc/array.c:241 invoked rcu_dereference_check() without protection!
> 
> Color me confused.  I cloned James Toy's git repository at
> git://zen-kernel.org/kernel/mmotm.git, and gitk claims that I am on tag
> 2010-04-05-16-09, which matches the string above.  But when I look at
> fs/proc/array.c near line 241, I see:

Andrew's -mm tree has 3 patches from Oleg Nesterov that hit that file, so the
code is different from what you show. Color *me* confused why your clone of
mmotm.git doesn't seem to contain them - I'm not sure how James Toy builds
that git tree.  Perhaps the tag is applied before those patches are - the
'mm.patch' that updates the Makefile with the version is usually in the
*middle* of the 'series' file.  What does HEAD of that tree look like?

My tree has:

/* needs ->siglock or rcu_read_lock() */
static void collect_sigign_sigcatch(struct task_struct *p, sigset_t *ign,
                                    sigset_t *catch)
{
        struct sighand_struct *sighand = rcu_dereference(p->sighand);

And that rcu_dereference() does it.

Oleg, looks like proc-make-collect_sigign_sigcatch-rcu-safe.patch is the
offender here, it added the line that causes the whinge.



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