linux-kernel - Re: [PATCH] [RESEND 2] Take over futex of dead task only if FUTEX

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Message-ID: <5088C1BE.9010203@linux.intel.com>
Date:	Wed, 24 Oct 2012 21:36:14 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] [RESEND 2] Take over futex of dead task only if FUTEX_WAITERS
 is not set



On 10/24/2012 11:08 AM, Thomas Gleixner wrote:
> On Wed, 24 Oct 2012, Siddhesh Poyarekar wrote:
> 
>>> Now there is a different solution to that problem. Do not look at the
>>> user space value at all and enforce a lookup of possibly available
>>> pi_state. If pi_state can be found, then the new incoming locker T3
>>> blocks on that pi_state and legitimately races with T2 to acquire the
>>> rt_mutex and the pi_state and therefor the proper ownership of the
>>> user space futex.
>>
>> That works. Thanks for the detailed explanation too.
> 
> Thanks for the reproducer and finding the trouble spot in the first
> place!


Absolutely, that was great. Siddhesh, any objection to this test being
incorporated into futextest?

http://git.kernel.org/?p=linux/kernel/git/dvhart/futextest.git;a=summary

> I'll queue that if Darren has no objections and mark it for stable as
> well.

I would mostly like to understand the stale waiters case you mentioned.
Otherwise, it seems sound - but changing what appears to be a workaround
for an undocumented cornercase in code this complex does make me a bit
nervous. I'd feel better if we could get Siddhesh's and this stale
waiters covered in futextest.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel
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