linux-kernel - Re: [PATCH v3 3/3] rcu: Use _full() API to debug synchronize

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Message-ID: <Z79-NAOoK-Vimgbr@pc636>
Date: Wed, 26 Feb 2025 21:48:52 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Boqun Feng <boqun.feng@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>, RCU <rcu@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Cheung Wall <zzqq0103.hey@...il.com>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v3 3/3] rcu: Use _full() API to debug synchronize_rcu()

Hello, Boqun.

> Hi Ulad,
> 
> On Tue, Feb 25, 2025 at 12:00:20PM +0100, Uladzislau Rezki (Sony) wrote:
> > Switch for using of get_state_synchronize_rcu_full() and
> > poll_state_synchronize_rcu_full() pair for debug a normal
> > synchronize_rcu() call.
> > 
> > Just using "not" full APIs to identify if a grace period
> > is passed or not might lead to a false kernel splat.
> > 
> 
> Could you provide detailed explanation on this? I.e. why is _full() is
> needed? I find the current commit message is a bit vague.
>
<snip>
rcu: Use _full() API to debug synchronize_rcu()

Switch for using of get_state_synchronize_rcu_full() and
poll_state_synchronize_rcu_full() pair to debug a normal
synchronize_rcu() call.

Just using "not" full APIs to identify if a grace period is
passed or not might lead to a false-positive kernel splat.

It can happen, because get_state_synchronize_rcu() compresses
both normal and expedited states into one single unsigned long
value, so a poll_state_synchronize_rcu() can miss GP-completion
when synchronize_rcu()/synchronize_rcu_expedited() concurrently
run.

To address this, switch to poll_state_synchronize_rcu_full() and
get_state_synchronize_rcu_full() APIs, which use separate variables
for expedited and normal states.
<snip>

Does it look better?

--
Uladzislau Rezki