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    SubjectRe: Q: perf_install_in_context/perf_event_enable are racy?
    On 01/27, Peter Zijlstra wrote:
    >
    > +void task_function_trampoline(void *data)
    > +{
    > + struct task_function_call *tfc = data;
    > + struct task_struct *p = tfc->p;
    > + struct rq *rq = this_rq();
    > +
    > +#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
    > + if (rq->in_ctxsw)
    > + return;
    > +#endif
    > +
    > + if (rq->curr != p)
    > + return;

    Yes, I think this should solve the problem.

    > prepare_task_switch(struct rq *rq, struct task_struct *prev,
    > struct task_struct *next)
    > {
    > +#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
    > + rq->in_ctxsw = 1;
    > +#endif
    > + sched_info_switch(prev, next);
    > + perf_event_task_sched_out(prev, next);
    > fire_sched_out_preempt_notifiers(prev, next);
    > prepare_lock_switch(rq, next);
    > prepare_arch_switch(next);
    > + trace_sched_switch(prev, next);
    > }

    Yes, I was wondering why schedule() calls perf_event_task_sched_out().
    This way the code looks more symmetrical/understandable.

    > /**
    > @@ -2823,6 +2860,7 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
    > perf_event_task_sched_in(current);
    > #ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
    > local_irq_enable();
    > + rq->in_ctxsw = 0;

    If we think that context_switch finishes here, probably it would be
    more clean to clear ->in_ctxsw before local_irq_enable().

    > #endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
    > finish_lock_switch(rq, prev);

    But, otoh, maybe finish_lock_switch() can clear in_ctxsw, it already
    checks __ARCH_WANT_INTERRUPTS_ON_CTXSW. Likewise, perhaps it can be
    set in prepare_lock_switch() which enables irqs.

    But this is cosmetic and up to you.

    Oleg.



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