On Tue, Nov 19, 2013 at 01:45:03PM -0800, Tim Chen wrote:
> On Tue, 2013-11-19 at 11:32 -0800, Paul E. McKenney wrote:
> > On Mon, Nov 11, 2013 at 08:57:27PM -0500, Waiman Long wrote:
> > > On 11/11/2013 04:17 PM, Tim Chen wrote:
> > > >>You could then augment that with [cmp]xchg_{acquire,release} as
> > > >>appropriate.
> > > >>
> > > >>>+/*
> > > >>> * In order to acquire the lock, the caller should declare a local node and
> > > >>> * pass a reference of the node to this function in addition to the lock.
> > > >>> * If the lock has already been acquired, then this will proceed to spin
> > > >>>@@ -37,15 +62,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> > > >>> node->locked = 0;
> > > >>> node->next = NULL;
> > > >>>
> > > >>>- prev = xchg(lock, node);
> > > >>>+ /* xchg() provides a memory barrier */
> > > >>>+ prev = xchg_acquire(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))
> > > >>>+ /*
> > > >>>+ * Wait until the lock holder passes the lock down.
> > > >>>+ * Using smp_load_acquire() provides a memory barrier that
> > > >>>+ * ensures subsequent operations happen after the lock is acquired.
> > > >>>+ */
> > > >>>+ while (!(smp_load_acquire(&node->locked)))
> > > >>> arch_mutex_cpu_relax();
> > > >An alternate implementation is
> > > > while (!ACCESS_ONCE(node->locked))
> > > > arch_mutex_cpu_relax();
> > > > smp_load_acquire(&node->locked);
> > > >
> > > >Leaving the smp_load_acquire at the end to provide appropriate barrier.
> > > >Will that be acceptable?
> > > >
> > > >Tim
> > >
> > > I second Tim's opinion. It will be help to have a
> > > smp_mb_load_acquire() function that provide a memory barrier with
> > > load-acquire semantic. I don't think we need one for store-release
> > > as that will not be in a loop.
> >
> > Hmmm... I guess the ACCESS_ONCE() in the smp_load_acquire() should
> > prevent it from being optimized away. But yes, you then end up with
> > an extra load on the critical lock hand-off patch. And something
> > like an smp_mb_acquire() could then be useful, although I believe
> > that on all current hardware smp_mb_acquire() emits the same code
> > as would an smp_mb_release():
> >
> > o barrier() on TSO systems such as x86 and s390.
> >
> > o lwsync instruction on powerpc. (Really old systems would
> > want a different instruction for smp_mb_acquire(), but let's
> > not optimize for really old systems.)
> >
> > o dmb instruction on ARM.
> >
> > o mf instruction on ia64.
> >
> > So how about an smp_mb_acquire_release() to cover both use cases?
>
> I guess we haven't addressed Will's preference to use wfe for ARM
> instead of doing a spin loop. I'll like some suggestions on how to
> proceed here. Should we do arch_mcs_lock and arch_mcs_unlock, which
> defaults to the existing mcs_lock and mcs_unlock code, but allow
> architecture specific implementation?
It would be nice to confine the architecture-specific pieces if at all
possible. For example, can the architecture-specific piece be confined
to the actual high-contention lock handoff?
Thanx, Paul
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