linux-kernel - Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize

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Message-ID: <Z8Ckb6spK35-Ez4U@pc636>
Date: Thu, 27 Feb 2025 18:44:15 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>,
	"Uladzislau Rezki (Sony)" <urezki@...il.com>,
	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 v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

On Thu, Feb 27, 2025 at 09:26:40AM -0800, Paul E. McKenney wrote:
> On Thu, Feb 27, 2025 at 09:12:39AM -0800, Boqun Feng wrote:
> > Hi Ulad,
> > 
> > I put these three patches into next (and misc.2025.02.27a) for some
> > testing, hopefully it all goes well and they can make it v6.15.
> > 
> > A few tag changed below:
> > 
> > On Thu, Feb 27, 2025 at 02:16:13PM +0100, Uladzislau Rezki (Sony) wrote:
> > > 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.
> > > 
> > > Link: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
> > 
> > I switch this into "Closes:" per checkpatch.
> > 
> > > Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
> > > Reported-by: cheung wall <zzqq0103.hey@...il.com>
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > 
> > You seem to forget add Paul's Reviewed-by, so I add it in rcu/next.
> > Would you or Paul double-check the Reviewed-by should be here?
> 
> I am good with keeping my Reviewed-by tags.
> 
Thanks Paul!

--
Uladzislau Rezki