linux-kernel - Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups

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Date:	Tue, 12 Aug 2014 14:39:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	linux-kernel@...r.kernel.org, riel@...hat.com, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB
 kthread wakeups

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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