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?
Tim
> This could be used to further optimize circular buffers, for example.
>
> Thanx, Paul
>
> > Peter, what do you think about adding that to your patch?
> >
> > -Longman
> >
>
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