Re: [PATCH 02/10] perf, trace: Use per-tracepoint-per-cpu hlist to track events

Frederic Weisbecker
2010-05-21T10:21:28Z

On Fri, May 21, 2010 at 12:19:09PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-05-21 at 12:13 +0200, Frederic Weisbecker wrote:
> > > I assumed that after probe unregister a tracepoint callback doesn't
> > > happen, which then guarantees we should never get !head.
>
> > I'm not sure about this. The tracepoints are called under rcu_read_lock(),
> > but there is not synchronize_rcu() after we unregister a tracepoint, which
> > means you can have a pending preempted one somewhere.
> >
> > There is a call_rcu that removes the callbacks, but that only protect
> > the callback themselves.
>
> Ah, ok, so we should do probe_unregister + synchronize_sched().
> That should ensure __DO_TRACE() doesn't call into it anymore.
>
> /me goes make a patch
>

Yep. But that also means we need to rcu_dereference_sched() to access
the per cpu list of events.

Thanks.