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

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Date:	Fri, 16 May 2014 13:55:59 -0400
From:	"Carlos O'Donell" <carlos@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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 05/14/2014 06:28 AM, Thomas Gleixner wrote:
> 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.

For the record the correct link is for POSIX Issue 7 (Issue 8 under development).

http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html

The Issue 7 version has a nice table :}

Cheers,
Carlos.

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