Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

[Paul E. McKenney]

On Wed, Mar 01, 2023 at 07:43:34PM +0100, Uros Bizjak wrote:
> On Wed, Mar 1, 2023 at 5:38 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Wed, 1 Mar 2023 11:28:47 +0100
> > Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >
> > > These benefits are the reason the change to try_cmpxchg was accepted
> > > also in the linear code elsewhere in the linux kernel, e.g. [2,3] to
> > > name a few commits, with a thumbs-up and a claim that the new code is
> > > actually *clearer* at the merge commit [4].
> >
> > I'll say it here too. I really like Joel's suggestion of having a
> > cmpxchg_success() that does not have the added side effect of modifying the
> > old variable.
> >
> > I think that would allow for the arch optimizations that you are trying to
> > achieve, as well as remove the side effect that might cause issues down the
> > road.
> 
> Attached patch implements this suggestion.

Please help me out here.

Why on earth are we even discussing making this change to code that
normally never executes?  Performance is not a consideration here.

What am I missing here?  Is there some sort of forward-progress
issue that this change addresses?

							Thanx, Paul

> Uros.

> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index b10b8349bb2a..229263ebba3b 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -709,6 +709,12 @@ static void print_cpu_stall(unsigned long gps)
>  	set_preempt_need_resched();
>  }
>  
> +#define cmpxchg_success(ptr, old, new)				\
> +({								\
> +	__typeof__(*(ptr)) __tmp = (old);			\
> +	try_cmpxchg((ptr), &__tmp, (new));			\
> +})
> +
>  static void check_cpu_stall(struct rcu_data *rdp)
>  {
>  	bool didstall = false;
> @@ -760,7 +766,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  	jn = jiffies + ULONG_MAX / 2;
>  	if (rcu_gp_in_progress() &&
>  	    (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
> -	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
> +	    cmpxchg_success(&rcu_state.jiffies_stall, js, jn)) {
>  
>  		/*
>  		 * If a virtual machine is stopped by the host it can look to
> @@ -778,7 +784,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  
>  	} else if (rcu_gp_in_progress() &&
>  		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
> -		   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
> +		   cmpxchg_success(&rcu_state.jiffies_stall, js, jn)) {
>  
>  		/*
>  		 * If a virtual machine is stopped by the host it can look to