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

[David Laight]

On Thu, 23 Jul 2026 18:04:30 +0800
Zqiang <qiang.zh...@linux.dev> 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