[lkml]   [2018]   [Mar]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
SubjectRe: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)
----- On Mar 28, 2018, at 8:50 AM, Peter Zijlstra 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 ?



Mathieu Desnoyers
EfficiOS Inc.

 \ /
  Last update: 2018-03-28 16:49    [W:0.140 / U:1.832 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site