Re: [RFC] rcu: Prevent expedite reporting within RCU read-side section

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Mar 06, 2018 at 09:42:05PM +0800, Boqun Feng wrote:
> On Tue, Mar 06, 2018 at 02:31:58PM +0900, Byungchul Park wrote:
> > Hello Paul and RCU folks,
> > 
> > I am afraid I correctly understand and fix it. But I really wonder why
> > sync_rcu_exp_handler() reports the quiescent state even in the case that
> > current task is within a RCU read-side section. Do I miss something?
> > 
> > If I correctly understand it and you agree with it, I can add more logic
> > which make it more expedited by boosting current or making it urgent
> > when we fail to report the quiescent state on the IPI.
> > 
> > ----->8-----
> > From 0b0191f506c19ce331a1fdb7c2c5a00fb23fbcf2 Mon Sep 17 00:00:00 2001
> > From: Byungchul Park <byungchul.park@lge.com>
> > Date: Tue, 6 Mar 2018 13:54:41 +0900
> > Subject: [RFC] rcu: Prevent expedite reporting within RCU read-side section
> > 
> > We report the quiescent state for this cpu if it's out of RCU read-side
> > section at the moment IPI was just fired during the expedite process.
> > 
> > However, current code reports the quiescent state even in the case:
> > 
> >    1) the current task is still within a RCU read-side section
> >    2) the current task has been blocked within the RCU read-side section
> 
> If this happens, the task will queue itself in
> rcu_preempt_note_context_switch() using rcu_preempt_ctxt_queue(). The gp
> kthread will wait for this task to dequeue itself. IOW, we have other
> mechanism to wait for this task other than bottom-up qs reporting tree.
> So I think we are fine here.

That is indeed the trick!  ;-)

							Thanx, Paul

> Regards,
> Boqun
> 
> > Since we don't get to the quiescent state yet in the case, we shouldn't
> > report it but check it another time.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > ---
> >  kernel/rcu/tree_exp.h | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 73e1d3d..cc69d14 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -731,13 +731,13 @@ static void sync_rcu_exp_handler(void *info)
> >  	/*
> >  	 * We are either exiting an RCU read-side critical section (negative
> >  	 * values of t->rcu_read_lock_nesting) or are not in one at all
> > -	 * (zero value of t->rcu_read_lock_nesting).  Or we are in an RCU
> > -	 * read-side critical section that blocked before this expedited
> > -	 * grace period started.  Either way, we can immediately report
> > -	 * the quiescent state.
> > +	 * (zero value of t->rcu_read_lock_nesting). We can immediately
> > +	 * report the quiescent state.
> >  	 */
> > -	rdp = this_cpu_ptr(rsp->rda);
> > -	rcu_report_exp_rdp(rsp, rdp, true);
> > +	if (t->rcu_read_lock_nesting <= 0) {
> > +		rdp = this_cpu_ptr(rsp->rda);
> > +		rcu_report_exp_rdp(rsp, rdp, true);
> > +	}
> >  }
> >  
> >  /**
> > -- 
> > 1.9.1
> >