On Mon, Feb 11, 2013 at 01:11:29AM +0530, Srivatsa S. Bhat wrote:
> On 02/09/2013 05:37 AM, Paul E. McKenney wrote:
> > On Tue, Jan 22, 2013 at 01:05:10PM +0530, Srivatsa S. Bhat wrote:
> >> Once stop_machine() is gone from the CPU offline path, we won't be able to
> >> depend on preempt_disable() to prevent CPUs from going offline from under us.
> >>
> >> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going offline,
> >> while invoking from atomic context.
> >>
> >> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> >
> > Would it make sense for get_online_cpus_atomic() to return the current
> > CPU number?
>
> Hmm, I'm not so sure. I tried to model it after get_online_cpus(), which doesn't
> return anything (for other reasons, of course..)
>
> Moreover, a function name like *_cpu_* returning the CPU number would be intuitive.
> But a name such as *_cpus_* (plural) returning a CPU number might appear confusing..
>
> And also I don't think it'll make a huge improvement in the callers.. (We might
> be better off avoiding an smp_processor_id() if possible, since this function could
> be called in very hot paths).. So I don't see a strong case for returning the
> CPU number. But let me know if you think it'll still be worth it for some reason...
I just noted a lot of two-line code sequences in your patch that would be
one line if the CPU number was returned. But I don't feel strongly about
it, so if people are OK with the current version, no problem.
Thanx, Paul
> > Looks good otherwise.
> >
>
> Thank you very much for the detailed review, Paul!
>
> Regards,
> Srivatsa S. Bhat
>
> >
> >> ---
> >>
> >> kernel/smp.c | 40 ++++++++++++++++++++++++++--------------
> >> 1 file changed, 26 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/kernel/smp.c b/kernel/smp.c
> >> index 29dd40a..f421bcc 100644
> >> --- a/kernel/smp.c
> >> +++ b/kernel/smp.c
> >> @@ -310,7 +310,8 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> >> * prevent preemption and reschedule on another processor,
> >> * as well as CPU removal
> >> */
> >> - this_cpu = get_cpu();
> >> + get_online_cpus_atomic();
> >> + this_cpu = smp_processor_id();
> >>
> >> /*
> >> * Can deadlock when called with interrupts disabled.
> >> @@ -342,7 +343,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> >> }
> >> }
> >>
> >> - put_cpu();
> >> + put_online_cpus_atomic();
> >>
> >> return err;
> >> }
> >> @@ -371,8 +372,10 @@ int smp_call_function_any(const struct cpumask *mask,
> >> const struct cpumask *nodemask;
> >> int ret;
> >>
> >> + get_online_cpus_atomic();
> >> /* Try for same CPU (cheapest) */
> >> - cpu = get_cpu();
> >> + cpu = smp_processor_id();
> >> +
> >> if (cpumask_test_cpu(cpu, mask))
> >> goto call;
> >>
> >> @@ -388,7 +391,7 @@ int smp_call_function_any(const struct cpumask *mask,
> >> cpu = cpumask_any_and(mask, cpu_online_mask);
> >> call:
> >> ret = smp_call_function_single(cpu, func, info, wait);
> >> - put_cpu();
> >> + put_online_cpus_atomic();
> >> return ret;
> >> }
> >> EXPORT_SYMBOL_GPL(smp_call_function_any);
> >> @@ -409,25 +412,28 @@ void __smp_call_function_single(int cpu, struct call_single_data *data,
> >> unsigned int this_cpu;
> >> unsigned long flags;
> >>
> >> - this_cpu = get_cpu();
> >> + get_online_cpus_atomic();
> >> +
> >> + this_cpu = smp_processor_id();
> >> +
> >> /*
> >> * Can deadlock when called with interrupts disabled.
> >> * We allow cpu's that are not yet online though, as no one else can
> >> * send smp call function interrupt to this cpu and as such deadlocks
> >> * can't happen.
> >> */
> >> - WARN_ON_ONCE(cpu_online(smp_processor_id()) && wait && irqs_disabled()
> >> + WARN_ON_ONCE(cpu_online(this_cpu) && wait && irqs_disabled()
> >> && !oops_in_progress);
> >>
> >> if (cpu == this_cpu) {
> >> local_irq_save(flags);
> >> data->func(data->info);
> >> local_irq_restore(flags);
> >> - } else {
> >> + } else if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) {
> >> csd_lock(data);
> >> generic_exec_single(cpu, data, wait);
> >> }
> >> - put_cpu();
> >> + put_online_cpus_atomic();
> >> }
> >>
> >> /**
> >> @@ -451,6 +457,8 @@ void smp_call_function_many(const struct cpumask *mask,
> >> unsigned long flags;
> >> int refs, cpu, next_cpu, this_cpu = smp_processor_id();
> >>
> >> + get_online_cpus_atomic();
> >> +
> >> /*
> >> * Can deadlock when called with interrupts disabled.
> >> * We allow cpu's that are not yet online though, as no one else can
> >> @@ -467,17 +475,18 @@ void smp_call_function_many(const struct cpumask *mask,
> >>
> >> /* No online cpus? We're done. */
> >> if (cpu >= nr_cpu_ids)
> >> - return;
> >> + goto out_unlock;
> >>
> >> /* Do we have another CPU which isn't us? */
> >> next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> >> if (next_cpu == this_cpu)
> >> - next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
> >> + next_cpu = cpumask_next_and(next_cpu, mask,
> >> + cpu_online_mask);
> >>
> >> /* Fastpath: do that cpu by itself. */
> >> if (next_cpu >= nr_cpu_ids) {
> >> smp_call_function_single(cpu, func, info, wait);
> >> - return;
> >> + goto out_unlock;
> >> }
> >>
> >> data = &__get_cpu_var(cfd_data);
> >> @@ -523,7 +532,7 @@ void smp_call_function_many(const struct cpumask *mask,
> >> /* Some callers race with other cpus changing the passed mask */
> >> if (unlikely(!refs)) {
> >> csd_unlock(&data->csd);
> >> - return;
> >> + goto out_unlock;
> >> }
> >>
> >> raw_spin_lock_irqsave(&call_function.lock, flags);
> >> @@ -554,6 +563,9 @@ void smp_call_function_many(const struct cpumask *mask,
> >> /* Optionally wait for the CPUs to complete */
> >> if (wait)
> >> csd_lock_wait(&data->csd);
> >> +
> >> +out_unlock:
> >> + put_online_cpus_atomic();
> >> }
> >> EXPORT_SYMBOL(smp_call_function_many);
> >>
> >> @@ -574,9 +586,9 @@ EXPORT_SYMBOL(smp_call_function_many);
> >> */
> >> int smp_call_function(smp_call_func_t func, void *info, int wait)
> >> {
> >> - preempt_disable();
> >> + get_online_cpus_atomic();
> >> smp_call_function_many(cpu_online_mask, func, info, wait);
> >> - preempt_enable();
> >> + put_online_cpus_atomic();
> >>
> >> return 0;
> >> }
> >>
>
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