Re: [PATCH 2/2] tracepoint: introduce *_noupdate APIs.

From: Ingo Molnar
Date: Thu Oct 30 2008 - 19:14:46 EST



* Mathieu Desnoyers <compudj@xxxxxxxxxxxxxxxxxx> wrote:

> * Mathieu Desnoyers (mathieu.desnoyers@xxxxxxxxxx) wrote:
> > * Lai Jiangshan (laijs@xxxxxxxxxxxxxx) wrote:
> > >
> > > new APIs separate tracepoint_probe_register(),
> > > tracepoint_probe_unregister() into 2 steps. The first step of them
> > > is just update tracepoint_entry, not connect or disconnect.
> > >
> > > this patch introduce tracepoint_probe_update_all() for update all.
> > >
> > > these APIs are very useful for registering a lots of probes
> > > but just update once only. and a very important thing is that
> > > *_noupdate APIs do not require module_mutex.
> > >
> > > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> >
> > [...]
> >
> > > +/**
> > > + * tracepoint_probe_update_all - update tracepoints
> > > + */
> > > +void tracepoint_probe_update_all(void)
> > > +{
> > > + LIST_HEAD(release_probes);
> > > + struct tp_probes *pos, *next;
> > > +
> > > + mutex_lock(&tracepoints_mutex);
> > > + if (!need_update) {
> > > + mutex_unlock(&tracepoints_mutex);
> > > + return;
> > > + }
> > > + if (!list_empty(&old_probes))
> > > + list_replace_init(&old_probes, &release_probes);
> > > + need_update = 0;
> > > + mutex_unlock(&tracepoints_mutex);
> > > +
> > > + tracepoint_update_probes();
> >
> > I think the read-side of this release_probes list should be protected by
> > the tracepoints_mutex too. Two concurrent tracepoint_probe_update_all()
> > could cause havoc here :
> >
> >
> > mutex_lock(&tracepoints_mutex);
> >
> > > + list_for_each_entry_safe(pos, next, &release_probes, u.list) {
> > > + list_del(&pos->u.list);
> > > + call_rcu_sched(&pos->u.rcu, rcu_free_old_probes);
> > > + }
> >
> > mutex_unlock(&tracepoints_mutex);
> >
> > ?
> >
> > The rest looks good.
> >
>
> Argh, forget it. LIST_HEAD(release_probes); is local to the function,
> there is nothing to protect here. My eyes thought they saw a "static"
> here for some reason. Night shift....
>
> The patch is good as-is.
>
> Thanks !
>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>

ok, i've applied both patches to tip/tracing/tracepoints:

57bc8ea: tracepoint: introduce *_noupdate APIs.
bbec237: tracepoint: simplification for tracepoints using RCU

thanks!

(Note, i had to resolve conflicts, hopefully i got the end result
right. Please double-check tip/master.)

Ingo
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