linux-kernel - Re: [PATCH RFC tip/core/rcu 11/12] rcu: fix race condition in synchronize_sched

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Date:	Thu, 11 Nov 2010 04:31:46 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, 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
Subject: Re: [PATCH RFC tip/core/rcu 11/12] rcu: fix race condition in
 synchronize_sched_expedited()

On Thu, Nov 11, 2010 at 10:10:33AM +0100, Tejun Heo wrote:
> Hello, Paul, Lai.
> 
> On 11/11/2010 05:20 AM, Paul E. McKenney wrote:
> > On Wed, Nov 10, 2010 at 04:56:32PM +0800, Lai Jiangshan wrote:
> >> On 11/09/2010 09:26 PM, Tejun Heo wrote:
> >>> Hello, Paul.
> >>>
> >>>
> >>> How about something like the following?  It's slightly bigger but I
> >>> think it's a bit easier to understand.  Thanks.
> >>
> >> Hello, Paul, Tejun,
> >>
> >> I think this approach is good and much better when several tasks
> >> call synchronize_sched_expedited() at the same time.
> > 
> > I am becoming more comfortable with it as well.  Tejun, what kind of
> > testing did you do?  Lai, could you please run it on your systems?
> 
> I just compile tested it (so no SOB).  Please feel free to take it and
> shape it into a proper patch.  Oh, I think we can drop both mb()'s at
> the top and bottom as both atomic_inc_return() and atomic_cmpxchg()
> imply full memory barrier.

Actually, the memory barriers are still one source of discomfort to me.
I am concerned about the path out of the function that skips the
atomic_cmpxchg(), which seem to happen if some concurrent invocation
advances the "done" counter past us before we get around to checking it.
I agree on the atomic_inc_return() upon entry to the function, though.

And this is going to need some serious testing either way.  ;-)

							Thanx, Paul
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