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    SubjectRe: WARNING: at kernel/rcutree.c:1550 __rcu_process_callbacks+0x46f/0x4b0()
    On Fri, Jul 20, 2012 at 04:11:00PM +0530, Srivatsa S. Bhat wrote:
    > On 07/19/2012 10:45 PM, Paul E. McKenney wrote:
    > > 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?
    >
    > Yes, that fixed it, thanks! :-)
    > Sorry for the noise...

    May I add your Tested-by?

    Thanx, Paul

    > Regards,
    > Srivatsa S. Bhat
    >
    > >
    > > ------------------------------------------------------------------------
    > >
    > > 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@linaro.org>
    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
    > >
    > > 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()))
    > > invoke_rcu_core();
    > >
    > > - /* 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())) {
    > > local_irq_restore(flags);
    > > return;
    > > }
    > >



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