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    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 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

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