Re: [PATCH] perf: Prevent recursion in ring buffer

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 12, 2018 at 09:33:17PM +0200, Jiri Olsa wrote:

>   # perf record -e 'sched:sched_switch,sched:sched_wakeup' perf bench sched messaging

> The reason for the corruptions are some of the scheduling tracepoints,
> that have __perf_task dfined and thus allow to store data to another
> cpu ring buffer:
> 
>   sched_waking
>   sched_wakeup
>   sched_wakeup_new
>   sched_stat_wait
>   sched_stat_sleep
>   sched_stat_iowait
>   sched_stat_blocked

> And then iterates events of the 'task' and store the sample
> for any task's event that passes tracepoint checks:
> 
>   ctx = rcu_dereference(task->perf_event_ctxp[perf_sw_context]);
> 
>   list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
>     if (event->attr.type != PERF_TYPE_TRACEPOINT)
>       continue;
>     if (event->attr.config != entry->type)
>       continue;
> 
>     perf_swevent_event(event, count, &data, regs);
>   }
> 
> Above code can race with same code running on another cpu,
> ending up with 2 cpus trying to store under the same ring
> buffer, which is not handled at the moment.

It can yes, however the only way I can see this breaking is if we use
!inherited events with a strict per-task buffer, but your record command
doesn't use that.

Now, your test-case uses inherited events, which would all share the
buffer, however IIRC inherited events require per-task-per-cpu buffers,
because there is already no guarantee the various tasks run on the same
CPU in the first place.

This means we _should_ write to the @task's local CPU buffer, and that
would work again.

Let me try and figure out where this is going wrong.