Re: [PATCH -rcu dev 3/3] RFC: rcu/tree: Read dynticks_nmi_nesting in advance

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Aug 15, 2019 at 10:53:11PM -0400, Joel Fernandes (Google) wrote:
> I really cannot explain this patch, but without it, the "else if" block
> just doesn't execute thus causing the tick's dep mask to not be set and
> causes the tick to be turned off.
> 
> I tried various _ONCE() macros but the only thing that works is this
> patch.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/tree.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 856d3c9f1955..ac6bcf7614d7 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -802,6 +802,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
>  {
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  	long incby = 2;
> +	int dnn = rdp->dynticks_nmi_nesting;

I believe the accidental sign extension / conversion from long to int was
giving me an illusion since things started working well. Changing the 'int
dnn' to 'long dnn' gives similar behavior as without this patch! At least I
know now. Please feel free to ignore this particular RFC patch while I debug
this more (over the weekend or early next week). The first 2 patches are
good, just ignore this one.

thanks,

 - Joel


>  
>  	/* Complain about underflow. */
>  	WARN_ON_ONCE(rdp->dynticks_nmi_nesting < 0);
> @@ -826,7 +827,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
>  
>  		incby = 1;
>  	} else if (tick_nohz_full_cpu(rdp->cpu) &&
> -		   !rdp->dynticks_nmi_nesting &&
> +		   !dnn &&
>  		   rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
>  		rdp->rcu_forced_tick = true;
>  		tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
> -- 
> 2.23.0.rc1.153.gdeed80330f-goog
>