Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Mar 28, 2018, at 8:50 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Mar 27, 2018 at 12:05:23PM -0400, Mathieu Desnoyers wrote:
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index fb5fc458547f..66b070444a7e 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -1249,6 +1249,7 @@ static inline void __set_task_cpu(struct task_struct *p,
>> unsigned int cpu)
>>  #endif
>>  	p->wake_cpu = cpu;
>>  #endif
>> +	rseq_migrate(p);
>>  }
> 
> I think you want that in set_task_cpu(), right next to nr_migrations++.

This would miss the __set_task_cpu() call from sched_fork() and wake_up_new_task().
Those cases are not accounted as explicit "migrations", but it does change the CPU
of the current task. So if for some weird reason userspace wants to fork() while in
a rseq critical section, we want to trigger a rseq restart.

Note that rseq_fork() implies rseq_preempt(), but userspace can request to
track only migrations for a given rseq critical section (by using the
RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT flag), so the rseq_preempt() in rseq_fork()
is not enough to restart if a migration between CPUs is done across a fork.

An alternative to this would be to call rseq_migrate() in rseq_fork().

Thoughts ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com