On Thu, 8 Feb 2024 11:25:50 +0100
Mete Durlu <me...@linux.ibm.com> wrote:
> I have been only able to reliably reproduce this issue when the system
> is under load from stressors. But I am not sure if it can be considered
> as *really stressed*.
>
> system : 8 cpus (4 physical cores)
> load : stress-ng --fanotify 1 (or --context 2)
> result : ~5/10 test fails
>
> of course as load increases test starts to fail more often, but a
> single stressor doesn't seem like much to me for a 4 core machine.
>
> after adding synchronize_rcu() + patch from Sven, I am no longer seeing
> failures with the setup above. So it seems like synchronize_rcu() did
> the trick(or at least it helps a lot) for the case described on the
> previous mail. I couldn't trigger the failure yet, not even with
> increased load(but now the test case takes > 5mins to finish :) ).
Right, it will definitely force the race window to go away.
Can you still trigger this issue with just Sven's patch and not this change?
-- Steve
>
> Here is the diff:
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> @@ -9328,10 +9328,12 @@ rb_simple_write(struct file *filp, const char
> __user *ubuf,
> val = 0; /* do nothing */
> } else if (val) {
> tracer_tracing_on(tr);
> + synchronize_rcu();
> if (tr->current_trace->start)
> tr->current_trace->start(tr);
> } else {
> tracer_tracing_off(tr);
> + synchronize_rcu();
> if (tr->current_trace->stop)
> tr->current_trace->stop(tr);
>
> Not 100% sure if these were the correct places to add them.
