proposed smp_rendezvous change

[Andriy Gapon]

on 15/05/2011 10:12 Andriy Gapon said the following:
> on 14/05/2011 18:25 John Baldwin said the following:
>> Hmmm, so this is not actually sufficient.  NetApp ran into a very similar race
>> with virtual CPUs in BHyVe.  In their case because virtual CPUs are threads that
>> can be preempted, they have a chance at a longer race.
>>
>> The problem that they see is that even though the values have been updated, the
>> next CPU to start a rendezvous can clear smp_rv_waiters[2] to zero before one of
>> the other CPUs notices that it has finished.
> 
> As a follow up to my previous question.  Have you noticed that in my patch no
> slave CPU actually waits/spins on smp_rv_waiters[2]?  It's always only master
> CPU (and under smp_ipi_mtx).
> 

Here's a cleaner version of my approach to the fix.
This one does not remove the initial wait on smp_rv_waiters[0] in
smp_rendezvous_action() and thus does not renumber all smp_rv_waiters[] members
and thus hopefully should be clearer.

Index: sys/kern/subr_smp.c
===================================================================
--- sys/kern/subr_smp.c	(revision 221943)
+++ sys/kern/subr_smp.c	(working copy)
@@ -110,7 +110,7 @@ static void (*volatile smp_rv_setup_func)(void *ar
 static void (*volatile smp_rv_action_func)(void *arg);
 static void (*volatile smp_rv_teardown_func)(void *arg);
 static void *volatile smp_rv_func_arg;
-static volatile int smp_rv_waiters[3];
+static volatile int smp_rv_waiters[4];

 /*
  * Shared mutex to restrict busywaits between smp_rendezvous() and
@@ -338,11 +338,15 @@ smp_rendezvous_action(void)

 	/* spin on exit rendezvous */
 	atomic_add_int(&smp_rv_waiters[2], 1);
-	if (local_teardown_func == smp_no_rendevous_barrier)
+	if (local_teardown_func == smp_no_rendevous_barrier) {
+		atomic_add_int(&smp_rv_waiters[3], 1);
                 return;
+	}
 	while (smp_rv_waiters[2] < smp_rv_ncpus)
 		cpu_spinwait();

+	atomic_add_int(&smp_rv_waiters[3], 1);
+
 	/* teardown function */
 	if (local_teardown_func != NULL)
 		local_teardown_func(local_func_arg);
@@ -377,6 +381,9 @@ smp_rendezvous_cpus(cpumask_t map,
 	/* obtain rendezvous lock */
 	mtx_lock_spin(&smp_ipi_mtx);

+	while (smp_rv_waiters[3] < smp_rv_ncpus)
+		cpu_spinwait();
+
 	/* set static function pointers */
 	smp_rv_ncpus = ncpus;
 	smp_rv_setup_func = setup_func;
@@ -385,6 +392,7 @@ smp_rendezvous_cpus(cpumask_t map,
 	smp_rv_func_arg = arg;
 	smp_rv_waiters[1] = 0;
 	smp_rv_waiters[2] = 0;
+	smp_rv_waiters[3] = 0;
 	atomic_store_rel_int(&smp_rv_waiters[0], 0);

 	/* signal other processors, which will enter the IPI with interrupts off */


-- 
Andriy Gapon