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

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Date:	Wed, 2 Apr 2014 20:30:23 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.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

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.

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?
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