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SubjectQ: perf_install_in_context/perf_event_enable are racy?
On 01/19, Oleg Nesterov wrote:
>
> Also. I believe there are more problems in perf_install_event(), but
> I need to recheck.

Help! I can't believe it can be so trivially wrong, but otoh I can't
understand how this can be correct.

So, ignoring details and !task case, __perf_install_in_context() does:

if (cpuctx->task_ctx || ctx->task != current)
return;

cpuctx->task_ctx = ctx;
event_sched_in(event);

Stupid question, what if this task has already passed
perf_event_exit_task() and thus it doesn't have ->perf_event_ctxp[] ?
Given that perf_event_context_sched_out() does nothing if !ctx, who
will event_sched_out() this event?

OK, even if I am right this is trivial, we just need the additional
check.



But, it seems, there is another problem. Forget about the exiting,
I can't understand why we can trust current in the code above.
With __ARCH_WANT_INTERRUPTS_ON_CTXSW schedule() does:

// sets cpuctx->task_ctx = NULL
perf_event_task_sched_out();

// enables irqs
prepare_lock_switch();


// updates current_task
switch_to();

What if IPI comes in the window before switch_to() ?

(the same questions for __perf_event_enable).

Oleg.



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