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" <paul...@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 <paul...@linux.vnet.ibm.com>
> > Reviewed-by: Josh Triplett <j...@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"
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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