On Tue, Aug 12, 2014 at 09:06:21AM -0700, Paul E. McKenney wrote:
> On Tue, Aug 12, 2014 at 11:03:21AM +0530, Amit Shah wrote:
[ . . . ]
> > I know of only virtio-console doing this (via userspace only,
> > though).
>
> As in userspace within the guest? That would not work. The userspace
> that the qemu is running in might. There is a way to extract ftrace info
> from crash dumps, so one approach would be "sendkey alt-sysrq-c", then
> pull the buffer from the resulting dump. For all I know, there might also
> be some script that uses the qemu "x" command to get at the ftrace buffer.
>
> Again, I cannot reproduce this, and I have been through the code several
> times over the past few days, and am not seeing it. I could start
> sending you random diagnostic patches, but it would be much better if
> we could get the trace data from the failure.
Hearing no objections, random patch #1. The compiler could in theory
cause trouble without this patch, so there is some possibility that
it is a fix.
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a2333d07f5d6..1dc72f523c4a 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2043,7 +2043,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool
force)
if (!ACCESS_ONCE(rdp_leader->nocb_kthread))
return;
if (!ACCESS_ONCE(rdp_leader->nocb_leader_wake) || force) {
- /* Prior xchg orders against prior callback enqueue. */
+ /* Prior smp_mb__after_atomic() orders against prior enqueue. */
ACCESS_ONCE(rdp_leader->nocb_leader_wake) = true;
wake_up(&rdp_leader->nocb_wq);
}
@@ -2072,6 +2072,7 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
ACCESS_ONCE(*old_rhpp) = rhp;
atomic_long_add(rhcount, &rdp->nocb_q_count);
atomic_long_add(rhcount_lazy, &rdp->nocb_q_count_lazy);
+ smp_mb__after_atomic(); /* Store *old_rhpp before _wake test. */
/* If we are not being polled and there is a kthread, awaken it ... */
t = ACCESS_ONCE(rdp->nocb_kthread);
--
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