linux-kernel - Re: [RFC PATCH 5/8] entry: Explicitly flush pending rcuog wakeup before last rescheduling points

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Message-ID: <20210111051318.GZ2743@paulmck-ThinkPad-P72>
Date:   Sun, 10 Jan 2021 21:13:18 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: Re: [RFC PATCH 5/8] entry: Explicitly flush pending rcuog wakeup
 before last rescheduling points

On Mon, Jan 11, 2021 at 01:40:14AM +0100, Frederic Weisbecker wrote:
> On Sat, Jan 09, 2021 at 03:05:33AM +0100, Frederic Weisbecker wrote:
> > Following the idle loop model, cleanly check for pending rcuog wakeup
> > before the last rescheduling point on resuming to user mode. This
> > way we can avoid to do it from rcu_user_enter() with the last resort
> > self-IPI hack that enforces rescheduling.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar<mingo@...nel.org>
> > Cc: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  kernel/entry/common.c |  6 ++++++
> >  kernel/rcu/tree.c     | 12 +++++++-----
> >  2 files changed, 13 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> > index 378341642f94..8f3292b5f9b7 100644
> > --- a/kernel/entry/common.c
> > +++ b/kernel/entry/common.c
> > @@ -178,6 +178,9 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> >  		/* Architecture specific TIF work */
> >  		arch_exit_to_user_mode_work(regs, ti_work);
> >  
> > +		/* Check if any of the above work has queued a deferred wakeup */
> > +		rcu_nocb_flush_deferred_wakeup();
> 
> So this needs to be moved to the IRQs disabled section, just a few lines later,
> otherwise preemption may schedule another task that in turn do call_rcu() and create
> new deferred wake up (thank Paul for the warning). Not to mention moving to
> another CPU with its own deferred wakeups to flush...
> 
> I'll fix that for the next version.

Ah, so it was not just my laptop dying, then!  ;-)

							Thanx, Paul