linux-kernel - Re: [RFC][PATCH 31/31] locking,qrwlock: Employ atomic_fetch_add

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Message-ID: <571A3444.3090703@hpe.com>
Date:	Fri, 22 Apr 2016 10:25:08 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<torvalds@...ux-foundation.org>, <mingo@...nel.org>,
	<tglx@...utronix.de>, <will.deacon@....com>,
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	<fengguang.wu@...el.com>
Subject: Re: [RFC][PATCH 31/31] locking,qrwlock: Employ atomic_fetch_add_acquire()

On 04/22/2016 05:04 AM, Peter Zijlstra wrote:
> The only reason for the current code is to make GCC emit only the
> "LOCK XADD" instruction on x86 (and not do a pointless extra ADD on
> the result), do so nicer.
>
> Signed-off-by: Peter Zijlstra (Intel)<peterz@...radead.org>
> ---
>   kernel/locking/qrwlock.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -93,7 +93,7 @@ void queued_read_lock_slowpath(struct qr
>   	 * that accesses can't leak upwards out of our subsequent critical
>   	 * section in the case that the lock is currently held for write.
>   	 */
> -	cnts = atomic_add_return_acquire(_QR_BIAS,&lock->cnts) - _QR_BIAS;
> +	cnts = atomic_fetch_add_acquire(_QR_BIAS,&lock->cnts);
>   	rspin_until_writer_unlock(lock, cnts);
>
>   	/*
>
>

Thanks for taking out this weirdness in the code.

Acked-by: Waiman Long <waiman.long@....com>