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

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Message-ID: <Z8IAqQ3dYdDsj5GN@pc636>
Date: Fri, 28 Feb 2025 19:30:01 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Uladzislau Rezki <urezki@...il.com>, Boqun Feng <boqun.feng@...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 Fri, Feb 28, 2025 at 10:25:58AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 28, 2025 at 06:08:19PM +0100, Uladzislau Rezki wrote:
> > On Fri, Feb 28, 2025 at 05:36:47PM +0100, Uladzislau Rezki wrote:
> > > On Fri, Feb 28, 2025 at 07:41:40AM -0800, Paul E. McKenney wrote:
> > > > On Thu, Feb 27, 2025 at 06:44:15PM +0100, Uladzislau Rezki wrote:
> > > > > 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!
> > > > 
> > > > Except that I got this from overnight testing of rcu/dev on the shared
> > > > RCU tree:
> > > > 
> > > > WARNING: CPU: 5 PID: 14 at kernel/rcu/tree.c:1636 rcu_sr_normal_complete+0x5c/0x80
> > > > 
> > > > I see this only on TREE05.  Which should not be too surprising, given
> > > > that this is the scenario that tests it.  It happened within five minutes
> > > > on all 14 of the TREE05 runs.
> > > > 
> > > Hm.. This is not fun. I tested this on my system and i did not manage to
> > > trigger this whereas you do. Something is wrong.
> > >
> > We have below code to start a new GP, if we detect that processing is
> > starved:
> > 
> > <snip>
> > /*
> >  * The "start_new_poll" is set to true, only when this GP is not able
> >  * to handle anything and there are outstanding users. It happens when
> >  * the rcu_sr_normal_gp_init() function was not able to insert a dummy
> >  * separator to the llist, because there were no left any dummy-nodes.
> >  *
> >  * Number of dummy-nodes is fixed, it could be that we are run out of
> >  * them, if so we start a new pool request to repeat a try. It is rare
> >  * and it means that a system is doing a slow processing of callbacks.
> >  */
> >   if (start_new_poll)
> >     (void) start_poll_synchronize_rcu();
> > <snip>
> > 
> > we do not use a _full() version, since we need to inform rcu-gp-kthread
> > to initiate a new GP.
> > 
> > Any thoughts?
> 
> My kneejerk not-to-be-trusted take is that it does not matter which type
> of grace period gets started so long as a grace period does get started.
> Presumably you have done the get_state_synchronize_rcu_full() before
> this point?
> 
Yes, of course. That code is located in the gp-kthread. get_state_synchronize_rcu_full()
is done before.

--
Uladzislau Rezki