Re: [PATCH tip/core/rcu 4/8] rcu: Ensure that ->rcu_urgent_qs is set before resched IPI

[Steven Rostedt]

On Wed,  2 Oct 2019 18:33:01 -0700
paulmck@xxxxxxxxxx wrote:

> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
> 
> The RCU-specific resched_cpu() function sends a resched IPI to the
> specified CPU, which can be used to force the tick on for a given
> nohz_full CPU.  This is needed when this nohz_full CPU is looping in the
> kernel while blocking the current grace period.  However, for the tick
> to actually be forced on in all cases, that CPU's rcu_data structure's
> ->rcu_urgent_qs flag must be set beforehand.  This commit therefore  
> causes rcu_implicit_dynticks_qs() to set this flag prior to invoking
> resched_cpu() on a holdout nohz_full CPU.

Should this be marked for stable?

-- Steve

> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8110514..0d83b19 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1073,6 +1073,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>  	if (tick_nohz_full_cpu(rdp->cpu) &&
>  		   time_after(jiffies,
>  			      READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) {
> +		WRITE_ONCE(*ruqp, true);
>  		resched_cpu(rdp->cpu);
>  		WRITE_ONCE(rdp->last_fqs_resched, jiffies);
>  	}