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...
> 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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