On Fri, 2010-05-21 at 12:21 +0200, Frederic Weisbecker wrote:
quoted text
> 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.

Why?

The per-cpu vars are allocated and freed in a fully serialized manner,
there should be no races what so ever.

--
unsubscribe notice
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/