linux-kernel - Re: sched: memory corruption on completing completions

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Message-ID: <54D463B6.2020808@linux.vnet.ibm.com>
Date:	Fri, 06 Feb 2015 12:18:22 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Davidlohr Bueso <dave@...olabs.net>,
	Sasha Levin <sasha.levin@...cle.com>,
	Waiman Long <Waiman.Long@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched: memory corruption on completing completions

On 02/06/2015 04:27 AM, Linus Torvalds wrote:
> On Thu, Feb 5, 2015 at 2:37 PM, Davidlohr Bueso <dave@...olabs.net> wrote:
>>>
>>> It is possible that the paravirt spinlocks could be saved by:
>>>
>>>   - moving the clearing of TICKET_SLOWPATH_FLAG into the fastpath locking code.
>>
>> Ouch, to avoid deadlocks they explicitly need the unlock to occur before
>> the slowpath tail flag is read.
>
> Well, just make the unlock do the actual real unlock operation
> ("fastpath"), leaving the TICKET_SLOWPATH_FLAG alone. The unlock path
> can *test* the flag and do whatever kicking it needs to get people
> started again, but not clear it again.
>

This is definitely a good idea, will think more on this.

(especially since any remote possibility of forgetting to wake up the 
lock-waiter would result in eventual hang of kvm guest).
Hopeful to come up with a solution soon.

/me agreeing with the fact that we did not have the 'lifetime' in mind
during the design :(.

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