Re: [PATCH 1/2] rcu: Delete a redundant check in check_cpu_stall()

[Paul E. McKenney]

On Fri, Jul 21, 2023 at 03:57:15PM +0800, thunder.leizhen@xxxxxxxxxxxxxxx wrote:
> From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> 
> j = jiffies;
> js = READ_ONCE(rcu_state.jiffies_stall);			(1)
> if (ULONG_CMP_LT(j, js))					(2)
> 	return;
> 
> if (cmpxchg(&rcu_state.jiffies_stall, js, jn) == js)		(3)
> 	didstall = true;
> 
> if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {	(4)
> 	jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
> 	WRITE_ONCE(rcu_state.jiffies_stall, jn);
> }
> 
> For ease of description, the pseudo code is extracted as above. First,
> assume that only one CPU is operating, the condition 'didstall' is true
> means that (3) succeeds. That is, the value of rcu_state.jiffies_stall
> must be 'jn'.
> 
> Then, assuming that another CPU is also operating at the same time, there
> are two cases:
> 1. That CPU sees the updated result at (1), it returns at (2).
> 2. That CPU does not see the updated result at (1), it fails at (3) and
>    cmpxchg returns jn. So that, didstall cannot be true.

The history behind this one is that there are races in which the stall
can end in the midst of check_cpu_stall().  For example, when the activity
of producing the stall warning breaks things loose.

And yes, long ago, I figured that if things had been static for so
many seconds, they were unlikely to change, and thus omitted any and
all synchronization.  The Linux kernel taught me better.  ;-)

							Thanx, Paul

> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> ---
>  kernel/rcu/tree_stall.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index cc884cd49e026a3..371713f3f7d15d9 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -794,7 +794,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  			rcu_ftrace_dump(DUMP_ALL);
>  		didstall = true;
>  	}
> -	if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
> +	if (didstall) {
>  		jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>  		WRITE_ONCE(rcu_state.jiffies_stall, jn);
>  	}
> -- 
> 2.25.1
>