linux-kernel - Re: [PATCH 1/2] smp: Non busy-waiting IPI queue

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Date:	Wed, 2 Apr 2014 12:01:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Kevin Hilman <khilman@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] smp: Non busy-waiting IPI queue

On Wed, Apr 02, 2014 at 08:30:23PM +0200, Frederic Weisbecker wrote:
> 2014-04-02 20:05 GMT+02:00 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > On Wed, Apr 02, 2014 at 06:26:05PM +0200, Frederic Weisbecker wrote:
> >> diff --git a/kernel/smp.c b/kernel/smp.c
> >> index 06d574e..bfe7b36 100644
> >> --- a/kernel/smp.c
> >> +++ b/kernel/smp.c
> >> @@ -265,6 +265,50 @@ int smp_call_function_single_async(int cpu, struct call_single_data *csd)
> >>  }
> >>  EXPORT_SYMBOL_GPL(smp_call_function_single_async);
> >>
> >> +void generic_smp_queue_function_single_interrupt(void *info)
> >> +{
> >> +     struct queue_single_data *qsd = info;
> >> +
> >> +     WARN_ON_ONCE(xchg(&qsd->pending, 0) != 1);
> >
> > I am probably missing something here, but shouldn't this function copy
> > *qsd to a local on-stack variable before doing the above xchg()?  What
> > prevents the following from happening?
> >
> > o       CPU 0 does smp_queue_function_single(), which sets ->pending
> >         and fills in ->func and ->data.
> >
> > o       CPU 1 takes IPI, invoking generic_smp_queue_function_single_interrupt().
> >
> > o       CPU 1 does xchg(), so that ->pending is now zero.
> >
> > o       An attempt to reuse the queue_single_data sees ->pending equal
> >         to zero, so the ->func and ->data is overwritten.
> >
> > o       CPU 1 calls the new ->func with the new ->data (or any of the other
> >         two possible unexpected outcomes), which might not be helpful to
> >         the kernel's actuarial statistics.
> >
> > So what am I missing?
> 
> Ah, I forgot to precise that the function must remain the same for all
> calls on a single qsd. And the data is always the qsd so this one can
> only stay stable. So that shouldn't be a problem.

I did indeed miss that particular constraint.  ;-)

> But you're right. The fact that we pass the function as an argument of
> smp_queue_function_single() suggests that we can pass a different
> function across various calls on a same qsd. So that's confusing.
> Perhaps changing smp_queue_function_single() such that it only takes
> the qsd as an argument would make that clearer? Then it's up to the
> caller to initialize the qsd with the constant function. I could
> define smp_queue_function_init() for that purpose. Or
> DEFINE_QUEUE_FUNCTION_DATA() for static initializers.
> 
> How does that sound?

Sounds good!

							Thanx, Paul

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