proposed smp_rendezvous change

This is a change in vein of what I've been doing in the xcpu branch and it's
supposed to fix the issue by the recent commit that (probably unintentionally)
stress-tests smp_rendezvous in TSC code.

Non-essential changes:
- ditch initial, and in my opinion useless, pre-setup rendezvous in
smp_rendezvous_action()
- shift smp_rv_waiters indexes by one because of the above

Essential changes (the fix):
- re-use freed smp_rv_waiters[2] to indicate that a slave/target is really fully
done with rendezvous (i.e. it's not going to access any members of smp_rv_*
pseudo-structure)
- spin on smp_rv_waiters[2] upon _entry_ to smp_rendezvous_cpus() to not re-use
the smp_rv_* pseudo-structure too early

Index: sys/kern/subr_smp.c
===================================================================
--- sys/kern/subr_smp.c	(revision 221835)
+++ sys/kern/subr_smp.c	(working copy)
_at__at_ -316,19 +316,14 _at__at_
 	void (*local_action_func)(void*)   = smp_rv_action_func;
 	void (*local_teardown_func)(void*) = smp_rv_teardown_func;

-	/* Ensure we have up-to-date values. */
-	atomic_add_acq_int(&smp_rv_waiters[0], 1);
-	while (smp_rv_waiters[0] < smp_rv_ncpus)
-		cpu_spinwait();
-
 	/* setup function */
 	if (local_setup_func != smp_no_rendevous_barrier) {
 		if (smp_rv_setup_func != NULL)
 			smp_rv_setup_func(smp_rv_func_arg);

 		/* spin on entry rendezvous */
-		atomic_add_int(&smp_rv_waiters[1], 1);
-		while (smp_rv_waiters[1] < smp_rv_ncpus)
+		atomic_add_int(&smp_rv_waiters[0], 1);
+		while (smp_rv_waiters[0] < smp_rv_ncpus)
                 	cpu_spinwait();
 	}

_at__at_ -337,12 +332,16 _at__at_
 		local_action_func(local_func_arg);

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

+	atomic_add_int(&smp_rv_waiters[2], 1);
+
 	/* teardown function */
 	if (local_teardown_func != NULL)
 		local_teardown_func(local_func_arg);
_at__at_ -377,6 +376,10 _at__at_
 	/* obtain rendezvous lock */
 	mtx_lock_spin(&smp_ipi_mtx);

+	/* Wait for any previous unwaited rendezvous to finish. */
+	while (atomic_load_acq_int(&smp_rv_waiters[2]) < ncpus)
+		cpu_spinwait();
+
 	/* set static function pointers */
 	smp_rv_ncpus = ncpus;
 	smp_rv_setup_func = setup_func;
_at__at_ -395,7 +398,7 _at__at_
 		smp_rendezvous_action();

 	if (teardown_func == smp_no_rendevous_barrier)
-		while (atomic_load_acq_int(&smp_rv_waiters[2]) < ncpus)
+		while (atomic_load_acq_int(&smp_rv_waiters[1]) < ncpus)
 			cpu_spinwait();

 	/* release lock */

-- 
Andriy Gapon