----- On Mar 28, 2018, at 10:59 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote: > On Wed, Mar 28, 2018 at 10:47:54AM -0400, Mathieu Desnoyers wrote: >> ----- On Mar 28, 2018, at 8:50 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx 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(). > > Correct; but since those are _new_ tasks they _SHOULD_ not have an > active RSEQ to begin with. As long as fork() can be issued from a rseq critical section, nothing actually prevents this. This is a fork(), not an exec(), so the new tasks may very well be going through a restartable sequence when fork() happens. > >> 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. > > If at all possible I would make it SIGSEGV when issueing SYSCALL()s from > within an RSEQ. What's the goal there ? rseq critical sections can technically do system calls if they wish. Why prevent this ? How would you handle signal handlers that issue system calls while nested on top of a rseq critical section in the userspace thread ? SIGSEGV on SYSCALLs will break this case. > >> An alternative to this would be to call rseq_migrate() in rseq_fork(). >> >> Thoughts ? > > Yes, don't try and support that at all. It's _insane_. Thomas told me those fork corner-cases should be correctly handled in a previous version of the patchset. I'm following his advice here. So either we disallow fork() within rseq critical sections completely with some kind of validation, or we need to provide a non-bogus behavior when this happens. Given that fork(2) is async-signal-safe, this means a signal handler can do a fork() while nested on top of a userspace thread's rseq critical section. So prohibiting fork() from being called over a rseq c.s. does not seem like something we can do here. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
