linux-kernel - Re: [PATCH] rcu: Fix opposite might_sleep() check in rcu_blocking_is

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Message-ID: <20221217051759.GK4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 16 Dec 2022 21:17:59 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc:     "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: Fix opposite might_sleep() check in
 rcu_blocking_is_gp()

On Sat, Dec 17, 2022 at 02:44:47AM +0000, Zhang, Qiang1 wrote:
> 
> On Thu, Dec 15, 2022 at 11:57:55AM +0800, Zqiang wrote:
> > Currently, if the system is in the RCU_SCHEDULER_INACTIVE state, invoke
> > synchronize_rcu_*() will implies a grace period and return directly,
> > so there is no sleep action due to waiting for a grace period to end,
> > but this might_sleep() check is the opposite. therefore, this commit
> > puts might_sleep() check in the correct palce.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> >
> >Queued for testing and review, thank you!
> >
> >I was under the impression that might_sleep() did some lockdep-based
> >checking, but I am unable to find it.  If there really is such checking,
> >that would be a potential argument for leaving this code as it is.
> >
> >
> >__might_sleep
> >   __might_resched(file, line, 0)
> >      rcu_sleep_check()
> >
> >Does it refer to this rcu_sleep_check() ?
> >
> >If so, when in the RCU_SCHEDULER_INACTIVE state,  the debug_lockdep_rcu_enabled() is always
> >return false, so the RCU_LOCKDEP_WARN() also does not produce an actual warning.
> 
> and when the system_state == SYSTEM_BOOTING, we just did  rcu_sleep_check()  and then  return.

Very good, thank you!

Thoughts from others?

							Thanx, Paul

> Thanks
> Zqiang
> 
> >Thanks
> >Zqiang
> >
> 
> >But in the meantime, full speed ahead!  ;-)
> >
> >						Thanx, Paul
> >
> > ---
> >  kernel/rcu/tree.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index ee8a6a711719..65f3dd2fd3ae 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3379,9 +3379,10 @@ void __init kfree_rcu_scheduler_running(void)
> >   */
> >  static int rcu_blocking_is_gp(void)
> >  {
> > -	if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE)
> > +	if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE) {
> > +		might_sleep();
> >  		return false;
> > -	might_sleep();  /* Check for RCU read-side critical section. */
> > +	}
> >  	return true;
> >  }
> >  
> > -- 
> > 2.25.1
> >