Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

Tim Chen
2013-10-01T21:16:34Z

On Tue, 2013-10-01 at 16:01 -0400, Waiman Long wrote:
> On 10/01/2013 12:48 PM, Tim Chen wrote:
> > On Mon, 2013-09-30 at 12:36 -0400, Waiman Long wrote:
> >> On 09/30/2013 12:10 PM, Jason Low wrote:
> >>> On Mon, 2013-09-30 at 11:51 -0400, Waiman Long wrote:
> >>>> On 09/28/2013 12:34 AM, Jason Low wrote:
> >>>>>> Also, below is what the mcs_spin_lock() and mcs_spin_unlock()
> >>>>>> functions would look like after applying the proposed changes.
> >>>>>>
> >>>>>> static noinline
> >>>>>> void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> >>>>>> {
> >>>>>> struct mcs_spin_node *prev;
> >>>>>>
> >>>>>> /* Init node */
> >>>>>> node->locked = 0;
> >>>>>> node->next = NULL;
> >>>>>>
> >>>>>> prev = xchg(lock, node);
> >>>>>> if (likely(prev == NULL)) {
> >>>>>> /* Lock acquired. No need to set node->locked since it
> >>>>>> won't be used */
> >>>>>> return;
> >>>>>> }
> >>>>>> ACCESS_ONCE(prev->next) = node;
> >>>>>> /* Wait until the lock holder passes the lock down */
> >>>>>> while (!ACCESS_ONCE(node->locked))
> >>>>>> arch_mutex_cpu_relax();
> >>>>>> smp_mb();
> >>>> I wonder if a memory barrier is really needed here.
> >>> If the compiler can reorder the while (!ACCESS_ONCE(node->locked)) check
> >>> so that the check occurs after an instruction in the critical section,
> >>> then the barrier may be necessary.
> >>>
> >> In that case, just a barrier() call should be enough.
> > The cpu could still be executing out of order load instruction from the
> > critical section before checking node->locked? Probably smp_mb() is
> > still needed.
> >
> > Tim
>
> But this is the lock function, a barrier() call should be enough to
> prevent the critical section from creeping up there. We certainly need
> some kind of memory barrier at the end of the unlock function.

I may be missing something. My understanding is that barrier only
prevents the compiler from rearranging instructions, but not for cpu out
of order execution (as in smp_mb). So cpu could read memory in the next
critical section, before node->locked is true, (i.e. unlock has been
completed). If we only have a simple barrier at end of mcs_lock, then
say the code on CPU1 is

mcs_lock
x = 1;
...
x = 2;
mcs_unlock

and CPU 2 is

mcs_lock
y = x;
...
mcs_unlock

We expect y to be 2 after the "y = x" assignment. But we
we may execute the code as

CPU1 CPU2

x = 1;
... y = x; ( y=1, out of order load)
x = 2
mcs_unlock
Check node->locked==true
continue executing critical section (y=1 when we expect y=2)

So we get y to be 1 when we expect that it should be 2. Adding smp_mb
after the node->locked check in lock code

ACCESS_ONCE(prev->next) = node;
/* Wait until the lock holder passes the lock down */
while (!ACCESS_ONCE(node->locked))
arch_mutex_cpu_relax();
smp_mb();

should prevent this scenario.

Thanks.
Tim

>
> -Longman
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