linux-kernel - Re: 3.14.0+/x86: lockdep and mutexes not getting along

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Date:	Thu, 10 Apr 2014 12:15:44 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Jason Low <jason.low2@...com>
Cc:	"Michael L. Semon" <mlsemon35@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: 3.14.0+/x86: lockdep and mutexes not getting along

On Wed, Apr 09, 2014 at 10:42:59PM -0700, Jason Low wrote:
> On Wed, 2014-04-09 at 15:19 +0300, Kirill A. Shutemov wrote:
> > On Sun, Apr 06, 2014 at 01:12:14AM -0400, Michael L. Semon wrote:
> > > Hi!  Starting early in this merge window for 3.15, lockdep has been
> > > giving me trouble.  Normally, a splat will happen, lockdep will shut
> > > itself off, and my i686 Pentium 4 PC will continue.  Now, after the
> > > splat, it will allow one key of input at either a VGA console or over
> > > serial.  After that, only the magic SysRq keys and KDB still work.
> > > File activity stops, and many processes are stuck in the D state.
> > > 
> > > Bisect brought me here:
> > > 
> > > root@...earer:/usr/src/kernel-git/linux# git bisect good
> > > 6f008e72cd111a119b5d8de8c5438d892aae99eb is the first bad commit
> > > commit 6f008e72cd111a119b5d8de8c5438d892aae99eb
> > > Author: Peter Zijlstra <peterz@...radead.org>
> > > Date:   Wed Mar 12 13:24:42 2014 +0100
> > > 
> > >     locking/mutex: Fix debug checks
> > > 
> > >     OK, so commit:
> > > 
> > >       1d8fe7dc8078 ("locking/mutexes: Unlock the mutex without the wait_lock")
> > > 
> > >     generates this boot warning when CONFIG_DEBUG_MUTEXES=y:
> > > 
> > >       WARNING: CPU: 0 PID: 139 at /usr/src/linux-2.6/kernel/locking/mutex-debug.c:82 debug_mutex_unlock+0x155/0x180() DEBUG_LOCKS_WARN_ON(lock->owner != current)
> > > 
> > >     And that makes sense, because as soon as we release the lock a
> > >     new owner can come in...
> > > 
> > >     One would think that !__mutex_slowpath_needs_to_unlock()
> > >     implementations suffer the same, but for DEBUG we fall back to
> > >     mutex-null.h which has an unconditional 1 for that.
> > > 
> > >     The mutex debug code requires the mutex to be unlocked after
> > >     doing the debug checks, otherwise it can find inconsistent
> > >     state.
> > > 
> > >     Reported-by: Ingo Molnar <mingo@...nel.org>
> > >     Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> > >     Cc: jason.low2@...com
> 
> Hello,
> 
> As a starting point, would either of you like to test the following
> patch to see if it fixes the issue? This patch essentially generates the
> same code as in older kernels in the debug case. This applies on top of
> kernels with both commits 6f008e72cd11 and 1d8fe7dc8078.

I'm not able to trigger the lockdep report with the patch applied so far.

-- 
 Kirill A. Shutemov
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