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SubjectRe: [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@redhat.com



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