Messages in this thread | | | Date | Wed, 2 Apr 2014 12:01:08 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/2] smp: Non busy-waiting IPI queue |
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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@linux.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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