linux-kernel - Re: [PATCH 2/2] tracepoint: introduce *

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Date:	Fri, 31 Oct 2008 00:14:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tracepoint: introduce *_noupdate APIs.


* Mathieu Desnoyers <compudj@...stal.dyndns.org> wrote:

> * Mathieu Desnoyers (mathieu.desnoyers@...ymtl.ca) wrote:
> > * Lai Jiangshan (laijs@...fujitsu.com) 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@...fujitsu.com>
> > 
> > [...]
> > 
> > > +/**
> > > + * 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@...ymtl.ca>

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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