Re: [PATCH tip/core/rcu 11/15] rcu: Break more call_rcu() deadlock involving scheduler and perf

[Peter Zijlstra]

On Thu, Aug 28, 2014 at 11:26:30AM -0700, Paul E. McKenney wrote:

> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index b6acb9340192..a1af86099a67 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2121,16 +2121,23 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
>  			trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
>  					    TPS("WakeEmpty"));
>  		} else {
> -			rdp->nocb_defer_wakeup = true;
> +			rdp->nocb_defer_wakeup = RCU_NOGP_WAKE;
>  			trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
>  					    TPS("WakeEmptyIsDeferred"));
>  		}
>  		rdp->qlen_last_fqs_check = 0;
>  	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
>  		/* ... or if many callbacks queued. */
> -		wake_nocb_leader(rdp, true);
> +		if (!irqs_disabled_flags(flags)) {
> +			wake_nocb_leader(rdp, true);
> +			trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
> +					    TPS("WakeOvf"));
> +		} else {
> +			rdp->nocb_defer_wakeup = RCU_NOGP_WAKE_FORCE;
> +			trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
> +					    TPS("WakeOvfIsDeferred"));
> +		}
>  		rdp->qlen_last_fqs_check = LONG_MAX / 2;
> -		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeOvf"));
>  	} else {
>  		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeNot"));
>  	}

Is it possible for the RCU_NOCP_WAKE write to overwrite a WAKE_FORCE ?
If not, why not? (Would make a good comment thereabouts).
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