linux-kernel - Re: [PATCH tip/core/rcu 0/6] Check for use of RCU from dyntick-idle mode

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Message-ID: <20110711170014.GI2245@linux.vnet.ibm.com>
Date:	Mon, 11 Jul 2011 10:00:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 0/6] Check for use of RCU from
 dyntick-idle mode

On Mon, Jul 11, 2011 at 06:44:08PM +0200, Frederic Weisbecker wrote:
> On Mon, Jul 11, 2011 at 09:38:48AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 11, 2011 at 06:03:32PM +0200, Frederic Weisbecker wrote:
> > > On Fri, Jul 08, 2011 at 08:43:31AM -0700, Paul E. McKenney wrote:
> > > > Hello!
> > > > 
> > > > This set of patches adds checks for use of RCU from "extended quiescent
> > > > states" such as dyntick-idle mode.  Such use is grossly illegal because
> > > > RCU by definition ignores CPUs that are in extended quiescent states.
> > > > In the case of dyntick-idle mode, the only way for RCU to avoid ignoring
> > > > such CPUs would be to wake them up periodically, which would defeat the
> > > > whole purpose of dyntick-idle mode.
> > > > 
> > > > The good news is that Frederic got this effort started.  The bad news is
> > > > that there are several cases where RCU read-side critical sections appear
> > > > in dyntick-idle mode.
> > > 
> > > Ok, let me send to you the patch that splits up rcu/tickless logic and I'll try
> > > to fix up what you reported to me in ppc.
> > 
> > Very good, thank you!
> > 
> > > BTW, are you aware of other cases? You mentioned "several" :)
> > 
> > PowerPC's hypercall-exit trace event will also cause this complaint.
> 
> Ok looking at this.

Oh, and I removed some of my RCU dyntick-idle trace events because
they triggered this warning.  This would of course be a problem regardless
of where the RCU dyntick-idle APIs were called from, so I just took the
approach of more carefully placing the trace events.

> > Plus I thought you saw some others.
> 
> Nope, mine were spurious. In my v1 rcu_dereference_check warned if rcu read lock
> wasn't held but didn't handle the rest of the conditional (another lock held or
> simply 1 in rcu_dereference_raw()).
> 
> In the v3, the one you applied, they legitimately disappeared.

OK, good to know!

							Thanx, Paul
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