Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()

[Valentin Schneider <valentin.schneider@arm.com>]

Hi,

On 10/08/21 10:42, Boqun Feng wrote:
> Hi,
>
> On Sat, Aug 07, 2021 at 01:58:05AM +0100, Valentin Schneider wrote:
>> Some areas use preempt_disable() + preempt_enable() to safely access
>> per-CPU data. The PREEMPT_RT folks have shown this can also be done by
>> keeping preemption enabled and instead disabling migration (and acquiring a
>> sleepable lock, if relevant).
>> 
>> Introduce a helper which checks whether the current task can safely access
>> per-CPU data, IOW if the task's context guarantees the accesses will target
>> a single CPU. This accounts for preemption, CPU affinity, and migrate
>> disable - note that the CPU affinity check also mandates the presence of
>> PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the
>> upcoming per-CPU access(es) unsafe.
>> 
>> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
>> ---
>>  include/linux/sched.h | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>> 
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index debc960f41e3..b77d65f677f6 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1715,6 +1715,16 @@ static inline bool is_percpu_thread(void)
>>  #endif
>>  }
>>  
>> +/* Is the current task guaranteed not to be migrated elsewhere? */
>> +static inline bool is_pcpu_safe(void)
>> +{
>> +#ifdef CONFIG_SMP
>> +	return !preemptible() || is_percpu_thread() || current->migration_disabled;
>> +#else
>> +	return true;
>> +#endif
>> +}
>
> I wonder whether the following can happen, say thread A is a worker
> thread for CPU 1, so it has the flag PF_NO_SETAFFINITY set.
>
> 	{ percpu variable X on CPU 2 is initially 0 }
>
> 	thread A
> 	========
>
> 	<preemption enabled>
> 	if (is_pcpu_safe()) { // nr_cpus_allowed == 1, so return true.
> 		<preempted>
> 		<hot unplug CPU 1>
> 			unbinder_workers(1); // A->cpus_mask becomes cpu_possible_mask
> 		<back to run on CPU 2>
> 		__this_cpu_inc(X);
> 		  tmp = X; // tmp == 0
> 		  <preempted>
> 		  <in thread B>
> 		  this_cpu_inc(X); // X becomes 1
> 		  <back to run A on CPU 2>
> 		  X = tmp + 1; // race!
> 	}
>
> if so, then is_percpu_thread() doesn't indicate is_pcpu_safe()?
>

You're absolutely right.

migrate_disable() protects the thread against being migrated due to
hotplug, but pure CPU affinity doesn't at all. kthread_is_per_cpu() doesn't
work either, because parking is not the only approach to hotplug for those
(e.g. per-CPU workqueue threads unbind themselves on hotplug, as in your
example).

One could hold cpus_read_lock(), but I don't see much point here. So that
has to be 

  return !preemptible() || current->migration_disabled;

Thanks!