linux-kernel - Re: [PATCH] ipc,sem: move restart loop to do_smart

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Date:	Wed, 22 May 2013 15:20:53 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, hhuang@...hat.com
Subject: Re: [PATCH] ipc,sem: move restart loop to do_smart_update

On Sun, 2013-05-19 at 18:32 -0400, Rik van Riel wrote:
> > 
> > One fix would be a loop in do_smart_update():
> > - first check the global queue
> > - then the per-semaphore queues
> > - if one of the per-semaphore queues made progress: check the global 
> > queue again
> > - if the global queue made progress: check the per semaphore queues again
> > ...
> 
> Would that be as simple as making do_smart_update() loop back to
> the top if update_queue on a single semaphore's queue returns
> a non-zero value (something was changed), and there are complex
> operations pending?

I've been looking at the code for a while and this approach seems quite
reasonable. I'd still like Manfred's feedback though. I ran pgbench and
your semop-multi program, nothing suspicious.

> ---8<---
> 
> Subject: ipc,sem: move restart loop to do_smart_update
> 
> A complex operation may be sleeping on a semaphore to become
> a certain value. A sleeping simple operation may turn the
> semaphore into that value.
> 
> Having the restart loop inside update_queue means we may be
> missing the complex operation (which lives on a different
> queue), and result in a semaphore lockup.
> 
> The lockup can be avoided by moving the restart loop into
> do_smart_update, so the list of pending complex operations
> will also be checked if required.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Reported-by: Manfred Spraul <manfred@...orfullife.com>

Acked-by: Davidlohr Bueso <davidlohr.bueso@...com>

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