On Fri, 2013-09-27 at 19:19 -0700, Paul E. McKenney wrote: > On Fri, Sep 27, 2013 at 04:54:06PM -0700, Jason Low wrote: > > On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney > > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > Yep. The previous lock holder's smp_wmb() won't keep either the compiler > > > or the CPU from reordering things for the new lock holder. They could for > > > example reorder the critical section to precede the node->locked check, > > > which would be very bad. > > > > Paul, Tim, Longman, > > > > How would you like the proposed changes below? > > Could you point me at what this applies to? I can find flaws looking > at random pieces, given a little luck, but at some point I need to look > at the whole thing. ;-) > > Thanx, Paul Jason's patch is on top of the following patchset: https://lkml.org/lkml/2013/9/26/674 Thanks. Tim > > > --- > > Subject: [PATCH] MCS: optimizations and barrier corrections > > > > Delete the node->locked = 1 assignment if the lock is free as it won't be used. > > > > Delete the smp_wmb() in mcs_spin_lock() and add a full memory barrier at the > > end of the mcs_spin_lock() function. As Paul McKenney suggested, "you do need a > > full memory barrier here in order to ensure that you see the effects of the > > previous lock holder's critical section." And in the mcs_spin_unlock(), move the > > memory barrier so that it is before the "ACCESS_ONCE(next->locked) = 1;". > > > > Signed-off-by: Jason Low <jason.low2@xxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > > --- > > include/linux/mcslock.h | 7 +++---- > > 1 files changed, 3 insertions(+), 4 deletions(-) > > > > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h > > index 20fd3f0..edd57d2 100644 > > --- a/include/linux/mcslock.h > > +++ b/include/linux/mcslock.h > > @@ -26,15 +26,14 @@ void mcs_spin_lock(struct mcs_spin_node **lock, > > struct mcs_spin_node *node) > > > > prev = xchg(lock, node); > > if (likely(prev == NULL)) { > > - /* Lock acquired */ > > - node->locked = 1; > > + /* Lock acquired. No need to set node->locked since it > > won't be used */ > > return; > > } > > ACCESS_ONCE(prev->next) = node; > > - smp_wmb(); > > /* Wait until the lock holder passes the lock down */ > > while (!ACCESS_ONCE(node->locked)) > > arch_mutex_cpu_relax(); > > + smp_mb(); > > } > > > > static void mcs_spin_unlock(struct mcs_spin_node **lock, struct > > mcs_spin_node *node) > > @@ -51,8 +50,8 @@ static void mcs_spin_unlock(struct mcs_spin_node > > **lock, struct mcs_spin_node *n > > while (!(next = ACCESS_ONCE(node->next))) > > arch_mutex_cpu_relax(); > > } > > - ACCESS_ONCE(next->locked) = 1; > > smp_wmb(); > > + ACCESS_ONCE(next->locked) = 1; > > } > > > > #endif > > -- > > 1.7.1 > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>