Author: kib
Date: Tue Jul 21 22:56:46 2015
New Revision: 285771
URL: https://svnweb.freebsd.org/changeset/base/285771
Log:
The smp_rendezvous_cpus() function should ensure that all accesses
done by the functions called on other CPUs, are visible to the caller.
Pair otherwise useless acquire on smp_rv_waiters[3] with a release add
to ensure synchronized with relation, which guarantees visibility.
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Modified:
head/sys/kern/subr_smp.c
Modified: head/sys/kern/subr_smp.c
==============================================================================
--- head/sys/kern/subr_smp.c Tue Jul 21 21:46:24 2015 (r285770)
+++ head/sys/kern/subr_smp.c Tue Jul 21 22:56:46 2015 (r285771)
@@ -455,8 +455,13 @@ smp_rendezvous_action(void)
* This means that no member of smp_rv_* pseudo-structure will be
* accessed by this target CPU after this point; in particular,
* memory pointed by smp_rv_func_arg.
+ *
+ * The release semantic ensures that all accesses performed by
+ * the current CPU are visible when smp_rendezvous_cpus()
+ * returns, by synchronizing with the
+ * atomic_load_acq_int(&smp_rv_waiters[3]).
*/
- atomic_add_int(&smp_rv_waiters[3], 1);
+ atomic_add_rel_int(&smp_rv_waiters[3], 1);
td->td_critnest--;
KASSERT(owepreempt == td->td_owepreempt,
@@ -522,6 +527,11 @@ smp_rendezvous_cpus(cpuset_t map,
* CPUs to finish the rendezvous, so that smp_rv_*
* pseudo-structure and the arg are guaranteed to not
* be in use.
+ *
+ * Load acquire synchronizes with the release add in
+ * smp_rendezvous_action(), which ensures that our caller sees
+ * all memory actions done by the called functions on other
+ * CPUs.
*/
while (atomic_load_acq_int(&smp_rv_waiters[3]) < ncpus)
cpu_spinwait();
_______________________________________________
svn-src-head@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/svn-src-head
To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
