| Message ID | 1346352988-32444-12-git-send-email-paulmck@linux.vnet.ibm.com |
|---|---|
| State | Accepted |
| Commit | fdab649b1aa732cd6e79654349088465cdff49af |
| Headers | show |
On Thu, Aug 30, 2012 at 11:56:25AM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paul.mckenney@linaro.org> > > The first memory barrier in __call_rcu() is supposed to order any > updates done beforehand by the caller against the actual queuing > of the callback. However, the second memory barrier (which is intended > to order incrementing the queue lengths before queuing the callback) > is also between the caller's updates and the queuing of the callback. > The second memory barrier can therefore serve both purposes. > > This commit therefore removes the first memory barrier. I don't see any such second memory barrier in __call_rcu(), at least not in current master. Right after this smp_mb(), __call_rcu() enqueues the callback and increments the queue length. Did you add a second memory barrier in some other patch that hasn't made it upstream yet? If so, could you note that patch dependency explicitly in the commit message? - Josh Triplett > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > --- > kernel/rcutree.c | 2 -- > 1 files changed, 0 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index e58097b..5b6709b 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -1923,8 +1923,6 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu), > head->func = func; > head->next = NULL; > > - smp_mb(); /* Ensure RCU update seen before callback registry. */ > - > /* > * Opportunistically note grace-period endings and beginnings. > * Note that we might see a beginning right after we see an > -- > 1.7.8 >
On Fri, Aug 31, 2012 at 11:30:35AM -0700, Josh Triplett wrote: > On Thu, Aug 30, 2012 at 11:56:25AM -0700, Paul E. McKenney wrote: > > From: "Paul E. McKenney" <paul.mckenney@linaro.org> > > > > The first memory barrier in __call_rcu() is supposed to order any > > updates done beforehand by the caller against the actual queuing > > of the callback. However, the second memory barrier (which is intended > > to order incrementing the queue lengths before queuing the callback) > > is also between the caller's updates and the queuing of the callback. > > The second memory barrier can therefore serve both purposes. > > > > This commit therefore removes the first memory barrier. > > I don't see any such second memory barrier in __call_rcu(), at least not > in current master. Right after this smp_mb(), __call_rcu() enqueues the > callback and increments the queue length. > > Did you add a second memory barrier in some other patch that hasn't made > it upstream yet? If so, could you note that patch dependency explicitly > in the commit message? Argh, nevermind. Looked at the wrong branch, not master. Looking at master, I do indeed see the second smp_mb(). Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c index e58097b..5b6709b 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -1923,8 +1923,6 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu), head->func = func; head->next = NULL; - smp_mb(); /* Ensure RCU update seen before callback registry. */ - /* * Opportunistically note grace-period endings and beginnings. * Note that we might see a beginning right after we see an