Re: [PATCH tip/core/rcu 8/9] rcu: Simplify _rcu_barrier() processing

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 08/20/2013 10:42 AM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit drops an unneeded ACCESS_ONCE() and simplifies an "our work
> is done" check in _rcu_barrier().  This applies feedback from Linus
> (https://lkml.org/lkml/2013/7/26/777) that he gave to similar code
> in an unrelated patch.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> ---
>  kernel/rcutree.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index c6a064a..612aff1 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2817,9 +2817,20 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  	 * transition.  The "if" expression below therefore rounds the old
>  	 * value up to the next even number and adds two before comparing.
>  	 */
> -	snap_done = ACCESS_ONCE(rsp->n_barrier_done);
> +	snap_done = rsp->n_barrier_done;
>  	_rcu_barrier_trace(rsp, "Check", -1, snap_done);
> -	if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
> +
> +	/*
> +	 * If the value in snap is odd, we needed to wait for the current
> +	 * rcu_barrier() to complete, then wait for the next one, in other
> +	 * words, we need the value of snap_done to be three larger than
> +	 * the value of snap.  On the other hand, if the value in snap is
> +	 * even, we only had to wait for the next rcu_barrier() to complete,
> +	 * in other words, we need the value of snap_done to be only two
> +	 * greater than the value of snap.  The "(snap + 3) & 0x1" computes

"(snap + 3) & 0x1"
==> "(snap + 3) & ~0x1"

> +	 * this for us (thank you, Linus!).
> +	 */
> +	if (ULONG_CMP_GE(snap_done, (snap + 3) & ~0x1)) {
>  		_rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
>  		smp_mb(); /* caller's subsequent code after above check. */
>  		mutex_unlock(&rsp->barrier_mutex);