linux-kernel - Re: [PATCH v2 05/13] rwsem: simplify rwsem_down_write

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Message-ID: <5154A8FF.4050504@redhat.com>
Date:	Thu, 28 Mar 2013 16:33:03 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/13] rwsem: simplify rwsem_down_write_failed

On 03/15/2013 06:54 AM, Michel Lespinasse wrote:
> When waking writers, we never grant them the lock - instead, they have
> to acquire it themselves when they run, and remove themselves from the
> wait_list when they succeed.
>
> As a result, we can do a few simplifications in rwsem_down_write_failed():
>
> - We don't need to check for !waiter.task since __rwsem_do_wake() doesn't
>    remove writers from the wait_list
>
> - There is no point releaseing the wait_lock before entering the wait loop,
>    as we will need to reacquire it immediately. We can change the loop so
>    that the lock is always held at the start of each loop iteration.
>
> - We don't need to get a reference on the task structure, since the task
>    is responsible for removing itself from the wait_list. There is no risk,
>    like in the rwsem_down_read_failed() case, that a task would wake up and
>    exit (thus destroying its task structure) while __rwsem_do_wake() is
>    still running - wait_lock protects against that.
>
> Signed-off-by: Michel Lespinasse <walken@...gle.com>

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

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