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

[Paul E. McKenney]

On Tue, Aug 20, 2013 at 05:48:01PM +0800, Lai Jiangshan wrote:
> On 08/20/2013 10:42 AM, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxxxxxxx>
> > Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > ---
> >  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"

Good catch, fixed!

							Thanx, Paul

> > +	 * 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);
> 

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