On Tue, 2013-11-05 at 18:37 +0000, Will Deacon wrote:
> On Tue, Nov 05, 2013 at 05:42:36PM +0000, Tim Chen wrote:
> > This patch corrects the way memory barriers are used in the MCS lock
> > and removes ones that are not needed. Also add comments on all barriers.
>
> Hmm, I see that you're fixing up the barriers, but I still don't completely
> understand how what you have is correct. Hopefully you can help me out :)
>
> > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > ---
> > include/linux/mcs_spinlock.h | 13 +++++++++++--
> > 1 files changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> > index 96f14299..93d445d 100644
> > --- a/include/linux/mcs_spinlock.h
> > +++ b/include/linux/mcs_spinlock.h
> > @@ -36,16 +36,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> > node->locked = 0;
> > node->next = NULL;
> >
> > + /* xchg() provides a memory barrier */
> > prev = xchg(lock, node);
> > if (likely(prev == NULL)) {
> > /* Lock acquired */
> > 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();
> > +
> > + /* Make sure subsequent operations happen after the lock is acquired */
> > + smp_rmb();
>
> Ok, so this is an smp_rmb() because we assume that stores aren't speculated,
> right? (i.e. the control dependency above is enough for stores to be ordered
> with respect to taking the lock)...
>
> > }
> >
> > /*
> > @@ -58,6 +61,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
> >
> > if (likely(!next)) {
> > /*
> > + * cmpxchg() provides a memory barrier.
> > * Release the lock by setting it to NULL
> > */
> > if (likely(cmpxchg(lock, node, NULL) == node))
> > @@ -65,9 +69,14 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
> > /* Wait until the next pointer is set */
> > while (!(next = ACCESS_ONCE(node->next)))
> > arch_mutex_cpu_relax();
> > + } else {
> > + /*
> > + * Make sure all operations within the critical section
> > + * happen before the lock is released.
> > + */
> > + smp_wmb();
>
> ...but I don't see what prevents reads inside the critical section from
> moving across the smp_wmb() here.
This is to prevent any read in next critical section from
creeping up before write in the previous critical section
has completed
e.g.
CPU 1 execute
mcs_lock
x = 1;
...
x = 2;
mcs_unlock
and CPU 2 execute
mcs_lock
y = x;
...
mcs_unlock
We expect y to be 2 after the "y = x" assignment. Without the proper
rmb in lock and wmb in unlock, y could be 1 for CPU 2 with
speculative read (i.e. before the x=2 assignment is completed).
Tim
>
> What am I missing?
>
> Will
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