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)
@@ -316,19 +316,14 @@
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();
}
@@ -337,12 +332,16 @@
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);
@@ -377,6 +376,10 @@
/* 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;
@@ -395,7 +398,7 @@
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