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

[Jiri Olsa]

On Thu, Sep 13, 2018 at 09:40:42AM +0200, Peter Zijlstra wrote:
> 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,

that's what perf record always does when monitoring task.. there's
an event/rb for each cpu and the given task

and all events for the task (sched:*) on given cpu share that single
cpu ring buffer via PERF_EVENT_IOC_SET_OUTPUT

> 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.