Re: [PATCH v3 16/45] rcu: Use cpu_is_offline_nocheck() to avoid false-positive warnings

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jun 28, 2013 at 01:25:17AM +0530, Srivatsa S. Bhat wrote:
> In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
> while being protected by a spinlock. At first, it appears as if we need to
> use the get/put_online_cpus_atomic() APIs to properly synchronize with CPU
> hotplug, once we get rid of stop_machine(). However, RCU has adequate
> synchronization with CPU hotplug, making that unnecessary. But since the
> locking details are non-trivial, it is hard to teach this to the rudimentary
> hotplug locking validator.
> 
> So use the _nocheck() variants of the cpu accessor functions to prevent false-
> positive warnings from the CPU hotplug debug code. Also, add a comment
> explaining the hotplug synchronization design used in RCU, so that its easy
> to see why it is justified to use the _nocheck() variants.
> 
> Cc: Dipankar Sarma <dipankar@in.ibm.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
> 
>  kernel/rcutree.c |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index cf3adc6..ced28a45 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -794,7 +794,17 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>  	if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
>  		return 0;  /* Grace period is not old enough. */
>  	barrier();
> -	if (cpu_is_offline(rdp->cpu)) {
> +
> +	/*
> +	 * It is safe to use the _nocheck() version of cpu_is_offline() here
> +	 * (to avoid false-positive warnings from CPU hotplug debug code),
> +	 * because:
> +	 * 1. rcu_gp_init() holds off CPU hotplug operations during grace
> +	 *    period initialization.
> +	 * 2. The current grace period has not ended yet.
> +	 * So it is safe to sample the offline state without synchronization.
> +	 */
> +	if (cpu_is_offline_nocheck(rdp->cpu)) {
>  		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
>  		rdp->offline_fqs++;
>  		return 1;
>