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

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Date:	Thu, 30 Oct 2008 01:39:49 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tracepoint: introduce *_noupdate APIs.

* 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>

> Mathieu
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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