linux-kernel - Re: [RFC patch 01/12] Kernel Tracepoints

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Date:	Tue, 08 Jul 2008 16:37:45 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC patch 01/12] Kernel Tracepoints

Hi Mathieu,

Masami Hiramatsu wrote:
[...]
>> +int tracepoint_probe_register(const char *name, void *probe)
>> +{
>> +	struct tracepoint_entry *entry;
>> +	int ret = 0;
>> +	void *old;
>> +
>> +	mutex_lock(&tracepoints_mutex);
>> +	entry = get_tracepoint(name);
>> +	if (!entry) {
>> +		entry = add_tracepoint(name);
>> +		if (IS_ERR(entry)) {
>> +			ret = PTR_ERR(entry);
>> +			goto end;
>> +		}
>> +	}
>> +	/*
>> +	 * If we detect that a call_rcu is pending for this tracepoint,
>> +	 * make sure it's executed now.
>> +	 */
>> +	if (entry->rcu_pending)
>> +		rcu_barrier();
>> +	old = tracepoint_entry_add_probe(entry, probe);
>> +	if (IS_ERR(old)) {
>> +		ret = PTR_ERR(old);
>> +		goto end;
>> +	}
>> +	mutex_unlock(&tracepoints_mutex);
>> +	tracepoint_update_probes();		/* may update entry */
>> +	mutex_lock(&tracepoints_mutex);
>> +	entry = get_tracepoint(name);
>> +	WARN_ON(!entry);

As I said in another patch, you might have to check
old != NULL here, because tracepoint_entry_add_probe() will
return NULL when you add a first probe to the entry.

>> +	entry->oldptr = old;
>> +	entry->rcu_pending = 1;
>> +	/* write rcu_pending before calling the RCU callback */
>> +	smp_wmb();
>> +#ifdef CONFIG_PREEMPT_RCU
>> +	synchronize_sched();	/* Until we have the call_rcu_sched() */
>> +#endif
>> +	call_rcu(&entry->rcu, free_old_closure);
>> +end:
>> +	mutex_unlock(&tracepoints_mutex);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(tracepoint_probe_register);
>> +
>> +/**
>> + * tracepoint_probe_unregister -  Disconnect a probe from a tracepoint
>> + * @name: tracepoint name
>> + * @probe: probe function pointer
>> + *
>> + * We do not need to call a synchronize_sched to make sure the probes have
>> + * finished running before doing a module unload, because the module unload
>> + * itself uses stop_machine(), which insures that every preempt disabled section
>> + * have finished.
>> + */
>> +int tracepoint_probe_unregister(const char *name, void *probe)
>> +{
>> +	struct tracepoint_entry *entry;
>> +	void *old;
>> +	int ret = -ENOENT;
>> +
>> +	mutex_lock(&tracepoints_mutex);
>> +	entry = get_tracepoint(name);
>> +	if (!entry)
>> +		goto end;
>> +	if (entry->rcu_pending)
>> +		rcu_barrier();
>> +	old = tracepoint_entry_remove_probe(entry, probe);
>> +	mutex_unlock(&tracepoints_mutex);
>> +	tracepoint_update_probes();		/* may update entry */
>> +	mutex_lock(&tracepoints_mutex);
>> +	entry = get_tracepoint(name);
>> +	if (!entry)
>> +		goto end;
>> +	entry->oldptr = old;
>> +	entry->rcu_pending = 1;
>> +	/* write rcu_pending before calling the RCU callback */
>> +	smp_wmb();
>> +#ifdef CONFIG_PREEMPT_RCU
>> +	synchronize_sched();	/* Until we have the call_rcu_sched() */
>> +#endif
>> +	call_rcu(&entry->rcu, free_old_closure);
>> +	remove_tracepoint(name);	/* Ignore busy error message */
>> +	ret = 0;
>> +end:
>> +	mutex_unlock(&tracepoints_mutex);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(tracepoint_probe_unregister);
>> +

On the other hand, tracepoint_entry_remove_probe() doesn't return NULL,
however, I think it might be better to introduce tracepoint_entry_free_old()
and simplify both of tracepoint_probe_register/unregister.

Thank you,
-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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