Re: [PATCH 2/5] perf: Support disable() after stop() on software events

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Jun 10, 2010 at 12:50:17PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-06-10 at 05:49 +0200, Frederic Weisbecker wrote:
> > If we call perf_event_stop() on a software event and then the
> > disable() pmu callback on them after that, we'll call twice
> > hlist_del_rcu() on the same hlist node and then bring a crash
> > by dereferencing LIST_POISON2.
> > 
> > Just use hlist_del_init_rcu() instead to fix this problem.
> > 
> > This preparates for new context exclusions.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Ingo Molnar <mingo@elte.hu>
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Stephane Eranian <eranian@google.com>
> > Cc: Cyrill Gorcunov <gorcunov@gmail.com>
> > Cc: Zhang Yanmin <yanmin_zhang@linux.intel.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > ---
> >  kernel/perf_event.c             |    2 +-
> >  kernel/trace/trace_event_perf.c |    2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> > index 5c004f7..2d818bc 100644
> > --- a/kernel/perf_event.c
> > +++ b/kernel/perf_event.c
> > @@ -4291,7 +4291,7 @@ static int perf_swevent_enable(struct perf_event *event)
> >  
> >  static void perf_swevent_disable(struct perf_event *event)
> >  {
> > -	hlist_del_rcu(&event->hlist_entry);
> > +	hlist_del_init_rcu(&event->hlist_entry);
> >  }
> >  
> >  static void perf_swevent_void(struct perf_event *event)
> > diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> > index 4799d70..7bc1f26 100644
> > --- a/kernel/trace/trace_event_perf.c
> > +++ b/kernel/trace/trace_event_perf.c
> > @@ -122,7 +122,7 @@ int perf_trace_enable(struct perf_event *p_event)
> >  
> >  void perf_trace_disable(struct perf_event *p_event)
> >  {
> > -	hlist_del_rcu(&p_event->hlist_entry);
> > +	hlist_del_init_rcu(&p_event->hlist_entry);
> >  }
> >  
> >  void perf_trace_destroy(struct perf_event *p_event)
> 
> 
> Ok, so then why did you need the first patch?


Because we need perf_event_stop/start to act on software events too.
The perf_event_enable,stop/start,perf_event_disable avoidance for
software is more an optimization that could be thought in another
patchset though.