linux-kernel - Re: [PATCH v2 12/13] rwsem: do not block readers at head of queue if other readers are active

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Date:	Thu, 28 Mar 2013 13:25:09 -0400
From:	Peter Hurley <peter@...leysoftware.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>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/13] rwsem: do not block readers at head of queue
 if other readers are active

On Fri, 2013-03-15 at 03:54 -0700, Michel Lespinasse wrote:
> This change fixes a race condition where a reader might determine it
> needs to block, but by the time it acquires the wait_lock the rwsem
> has active readers and no queued waiters.
> 
> In this situation the reader can just in parallel with the existing active
                                      ^^^
                                     start ?

> readers; it does not need to block until the active readers complete.
> 
> Thanks to Peter Hurley for noticing this possible race.
> 
> Signed-off-by: Michel Lespinasse <walken@...gle.com>
> 
> ---
>  lib/rwsem.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 09bf03e7808c..4e4c8893dc00 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -162,8 +162,14 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
>  	/* we're now waiting on the lock, but no longer actively locking */
>  	count = rwsem_atomic_update(adjustment, sem);
                      ^^^^^^^^^
>  
> -	/* If there are no active locks, wake the front queued process(es). */
> -	if (!(count & RWSEM_ACTIVE_MASK))
> +	/* If there are no active locks, wake the front queued process(es).
> +	 *
> +	 * If there are no writers and we are first in the queue,
> +	 * wake our own waiter to join the existing active readers !
> +	 */
> +	if (count == RWSEM_WAITING_BIAS ||
> +	    (count > RWSEM_WAITING_BIAS &&
> +	     adjustment != -RWSEM_ACTIVE_READ_BIAS))
>  		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);

Thanks for fixing this.


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