Messages in this thread Patch in this message |  | | Date | Wed, 28 Mar 2018 11:14:05 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12) |
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----- On Mar 28, 2018, at 10:59 AM, Peter Zijlstra peterz@infradead.org 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@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(). > > 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
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