Re: [PATCH] rcu: Reduce stack usage in show_rcu_gp_kthreads()

[David Laight]

On Thu, 23 Jul 2026 18:04:30 +0800
Zqiang <qiang.zhang@xxxxxxxxx> wrote:

> When CONFIG_KASAN=y and CONFIG_KASAN_STACK=y builds, the
> show_rcu_gp_kthreads() exceeds the 1024-byte frame-size limit:
> 
...
>  	rcu_for_each_node_breadth_first(rnp) {
>  		if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
>  		    !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
>  		    !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
>  			continue;
> -		pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
> -			rnp->grplo, rnp->grphi,
> -			(long)data_race(READ_ONCE(rnp->gp_seq)),
> -			(long)data_race(READ_ONCE(rnp->gp_seq_needed)),
> -			data_race(READ_ONCE(rnp->qsmask)),
> -			".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
> -			".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
> -			".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
> -			".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
> -			data_race(READ_ONCE(rnp->n_boosts)));

Isn't that code carefully reading all the values twice?

Also the "ab"[!!val] generates far worse code than the more obvious (val ? 'a' : 'b').
For the former gcc indexes a constant string, the latter is done using arithmetic.

I'm sure there is a good reason for data_race(READ_ONCE(xxx)) ...

	David