linux-kernel - Re: [patch 0/3] futex/rtmutex: Fix issues exposed by trinity

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Date:	Wed, 14 May 2014 12:28:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Carlos O'Donell <carlos@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Clark Williams <williams@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Roland McGrath <roland@...k.frob.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch 0/3] futex/rtmutex: Fix issues exposed by trinity

On Wed, 14 May 2014, Peter Zijlstra wrote:
> On Wed, May 14, 2014 at 11:53:44AM +0200, Thomas Gleixner wrote:
> > > What error would we return?
> > >
> > > This particular case is a serious error for which we have no good error code
> > > to return to userspace. It's an implementation defect, a bug, we should probably
> > > assert instead of pausing.
> > 
> > Errm.
> > 
> > http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_mutex_lock.html
> > 
> >  The pthread_mutex_lock() function may fail if:
> > 
> >   [EDEADLK]
> >     The current thread already owns the mutex. 
> > 
> > That's a exactly the error code, which the kernel returns when it
> > detects a deadlock. 
> > 
> > And glibc returns EDEADLK at a lot of places already. So in that case
> > it's not a serious error? Because it's detected by glibc. You can't be
> > serious about that.
> > 
> > So why is a kernel detected deadlock different? Because it detects not
> > only AA, it detects ABBA and more. But it's still a dead lock. And
> > while posix spec only talks about AA, it's the very same issue.
> > 
> > So why not propagate this to the caller so he gets an alert right away
> > instead of letting him attach a debugger, and scratch his head and
> > lookup glibc source to find out why the hell glibc called pause.
> 
>   http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html

Yuck. I should not have used the first link Gurgle brought up.
 
>   The pthread_mutex_lock() function may fail if:
> 
>   [EDEADLK]
>       A deadlock condition was detected or the current thread already owns the mutex.
> 
> Which is explicitly wider than the AA recursion and fully supports the
> full lock graph traversal we do.
 
Definitely. It's what the kernel does :)

Thanks,

	tglx
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