From: David Laight <david.laight.linux@gmail.com>
To: Zqiang <qiang.zhang@linux.dev>
Cc: paulmck@kernel.org, frederic@kernel.org,
neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com,
urezki@gmail.com, boqun@kernel.org, rcu@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: Reduce stack usage in show_rcu_gp_kthreads()
Date: Fri, 24 Jul 2026 14:20:48 +0100 [thread overview]
Message-ID: <20260724142048.53e392a9@pumpkin> (raw)
In-Reply-To: <20260723100430.17813-1-qiang.zhang@linux.dev>
On Thu, 23 Jul 2026 18:04:30 +0800
Zqiang <qiang.zhang@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
next prev parent reply other threads:[~2026-07-24 13:20 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-23 10:04 [PATCH] rcu: Reduce stack usage in show_rcu_gp_kthreads() Zqiang
2026-07-23 18:30 ` Paul E. McKenney
2026-07-24 13:20 ` David Laight [this message]
2026-07-24 18:11 ` Paul E. McKenney
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