From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com,
edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com,
sbw@mit.edu
Subject: Re: [PATCH tip/core/rcu 8/9] rcu: Simplify _rcu_barrier() processing
Date: Tue, 20 Aug 2013 11:50:06 -0700 [thread overview]
Message-ID: <20130820185006.GM29406@linux.vnet.ibm.com> (raw)
In-Reply-To: <52133B51.3020007@cn.fujitsu.com>
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@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"
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);
>
next prev parent reply other threads:[~2013-08-20 18:50 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-20 2:41 [PATCH tip/core/rcu 0/9] v2 Fixes for 3.12 Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 1/9] rcu: Expedite grace periods during suspend/resume Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 2/9] rcu: Simplify debug-objects fixups Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 3/9] debugobjects: Make debug_object_activate() return status Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 4/9] rcu: Make call_rcu() leak callbacks for debug-object errors Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 5/9] rcu: Avoid redundant grace-period kthread wakeups Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 6/9] rculist: list_first_or_null_rcu() should use list_entry_rcu() Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 7/9] rcu: Select IRQ_WORK from TREE_PREEMPT_RCU Paul E. McKenney
2013-08-20 2:42 ` [PATCH tip/core/rcu 8/9] rcu: Simplify _rcu_barrier() processing Paul E. McKenney
2013-08-20 9:48 ` Lai Jiangshan
2013-08-20 18:50 ` Paul E. McKenney [this message]
2013-08-20 2:42 ` [PATCH tip/core/rcu 9/9] jiffies: Avoid undefined behavior from signed overflow Paul E. McKenney
2013-08-20 9:58 ` [PATCH tip/core/rcu 1/9] rcu: Expedite grace periods during suspend/resume Lai Jiangshan
2013-08-20 18:42 ` Paul E. McKenney
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