linux-kernel - Re: [PATCH v2] ipc/sem.c: fix lockup, restore FIFO behavior

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Date:	Sat, 25 May 2013 13:55:47 -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 v2] ipc/sem.c: fix lockup, restore FIFO behavior

On 05/25/2013 11:16 AM, Manfred Spraul wrote:
> The double coward solution:
> - wakeup stays FIFO
> - fast switch back to per-semaphore spinlock mode
>
> The patch
> a) fixes a lockup due to a missing restart.
> b) makes the wakeups again FIFO (as linux <= 3.0.9)
> c) tries to limit the time while in global lock mode as much
>     as possible. (same as linux-3.0.10-rc1)
>
> Changes:
> - the wait-for-zero operations are moved into seperate lists. Thus they can
>    be checked seperately, without rescanning the whole queue.
> - If a complex operation must sleep, then all pending change operations are
>    moved into the global queue. This allows to keep everything FIFO.
> - When all complex operations have completed, the simple ops are moved
>    back into the per-semaphore queues.
>
> Advantage:
> - FIFO. Dropping FIFO is a user visible change, and I'm a coward.

I am still not entirely convinced that FIFO is worthwhile,
but the code looks correct to me.

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

> - simpler check_restart logic.
> - Efficient handling of wait-for-zero semops, both simple and complex.
> - Fewer restarts in update_queue(), because pending wait-for-zero do not
>    force a restart anymore.
>
> Other changes:
> - try_atomic_semop() also performs the semop. Thus rename the function.
>
> It passes tests with qemu, but not boot-tested due to EFI problems.
>
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>

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