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

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Message-ID: <20130627201246.GA3828@linux.vnet.ibm.com>
Date:	Thu, 27 Jun 2013 13:12:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, rusty@...tcorp.com.au, mingo@...nel.org,
	akpm@...ux-foundation.org, namhyung@...nel.org, walken@...gle.com,
	vincent.guittot@...aro.org, laijs@...fujitsu.com,
	David.Laight@...lab.com, rostedt@...dmis.org,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	sbw@....edu, fweisbec@...il.com, zhong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH v3 16/45] rcu: Use cpu_is_offline_nocheck() to avoid
 false-positive warnings

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@...ibm.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Reviewed-by: Paul E. McKenney <paulmck@...ux.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;
> 

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