linux-kernel - Re: [PATCH v2 06/13] rwsem: more agressive lock stealing in rwsem_down_write

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Message-ID: <5154AE1E.6000007@redhat.com>
Date:	Thu, 28 Mar 2013 16:54:54 -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 06/13] rwsem: more agressive lock stealing in rwsem_down_write_failed

On 03/15/2013 06:54 AM, Michel Lespinasse wrote:
> Some small code simplifications can be achieved by doing more agressive
> lock stealing:
>
> - When rwsem_down_write_failed() notices that there are no active locks
>    (and thus no thread to wake us if we decided to sleep), it used to wake
>    the first queued process. However, stealing the lock is also sufficient
>    to deal with this case, so we don't need this check anymore.
>
> - In try_get_writer_sem(), we can steal the lock even when the first waiter
>    is a reader. This is correct because the code path that wakes readers is
>    protected by the wait_lock. As to the performance effects of this change,
>    they are expected to be minimal: readers are still granted the lock
>    (rather than having to acquire it themselves) when they reach the front
>    of the wait queue, so we have essentially the same behavior as in
>    rwsem-spinlock.
>
> Signed-off-by: Michel Lespinasse <walken@...gle.com>

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

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