Re: [RFC][PATCH 31/31] locking,qrwlock: Employ atomic_fetch_add_acquire()

[Waiman Long]

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@xxxxxxxxxxxxx>
> ---
>   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@xxxxxxx>