linux-kernel - Re: [PATCH 1/4] ipc/sem.c: Fix missing wakeups in do_smart_update

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Date:	Mon, 27 May 2013 13:51:21 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Manfred Spraul <manfred@...orfullife.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>, hhuang@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/4] ipc/sem.c: Fix missing wakeups in do_smart_update_queue()

On 05/26/2013 05:08 AM, Manfred Spraul wrote:
> do_smart_update_queue() is called when an operation
> (semop, semctl(SETVAL), semctl(SETALL), ...) modified the array.
> It must check which of the sleeping tasks can proceed.
>
> do_smart_update_queue() missed a few wakeups:
> - if a sleeping complex op was completed, then all per-semaphore queues
>    must be scanned - not only those that were modified by *sops
> - if a sleeping simple op proceeded, then the global queue
>    must be scanned again
>
> And:
> - the test for "|sops == NULL) before scanning the global
>    queue is not required: If the global queue is empty, then
>    it doesn't need to be scanned - regardless of the reason
>    for calling do_smart_update_queue()
>
> The patch is not optimized, i.e. even completing a wait-for-zero
> operation causes a rescan. This is done to keep the patch as simple as
> possible.
> Avoiding unnecessary scans is implemented in the following patches.
>
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>

Very much not optimized, but we need to fix the regression.

Acked-by: Rik van Riel <riel@...hat.com>

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