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

[Srivatsa S. Bhat]

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...

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@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()))
>  		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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