Re: WARNING: at kernel/rcutree.c:1550__rcu_process_callbacks+0x46f/0x4b0()

From: Paul E. McKenney
Date: Thu Jul 19 2012 - 13:26:30 EST

On Thu, Jul 19, 2012 at 05:39:30PM +0530, Srivatsa S. Bhat wrote:
> Hi Paul,
> While running a CPU hotplug stress test on v3.5-rc7+
> (mainline commit 8a7298b7805ab) I hit this warning.
> I haven't tried to debug this yet...
> Line number 1550 maps to:
> WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
> inside rcu_do_batch().

Hello, Srivatsa,

I believe that you need commit a16b7a69 (Prevent __call_rcu() from
invoking RCU core on offline CPUs), which is currently in -tip, queued
for 3.6. Please see below for the patch.

Does this help?

Thanx, Paul


rcu: Prevent __call_rcu() from invoking RCU core on offline CPUs

The __call_rcu() function will invoke the RCU core, for example, if
it detects that the current CPU has too many callbacks. However, this
can happen on an offline CPU that is on its way to the idle loop, in
which case it is an error to invoke the RCU core, and the excess callbacks
will be adopted in any case. This commit therefore adds checks to
__call_rcu() for running on an offline CPU, refraining from invoking
the RCU core in this case.

Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a4a9c91..ceaa959 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1904,11 +1904,11 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
* If called from an extended quiescent state, invoke the RCU
* core in order to force a re-evaluation of RCU's idleness.
- if (rcu_is_cpu_idle())
+ if (rcu_is_cpu_idle() && cpu_online(smp_processor_id()))

- /* If interrupts were disabled, don't dive into RCU core. */
- if (irqs_disabled_flags(flags)) {
+ /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
+ if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id())) {

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