linux-kernel - Re: [PATCH 3/3] tracing: Add 2 new funcs. for kernel access to Ftrace instances.

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Date:   Wed, 12 Jun 2019 08:12:29 -0700
From:   Divya Indi <divya.indi@...cle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Srinivas Eeda <srinivas.eeda@...cle.com>
Subject: Re: [PATCH 3/3] tracing: Add 2 new funcs. for kernel access to Ftrace
 instances.

Hi Steven,

Thanks for taking the time to review. Please find my comments inline.

On 6/8/19 2:51 PM, Steven Rostedt wrote:
> On Tue,  4 Jun 2019 17:42:05 -0700
> Divya Indi <divya.indi@...cle.com> wrote:
>
>> Adding 2 new functions -
>> 1) trace_array_lookup : Look up and return a trace array, given its
>> name.
>> 2) trace_array_set_clr_event : Enable/disable event recording to the
>> given trace array.
>>
>> Signed-off-by: Divya Indi <divya.indi@...cle.com>
>> ---
>>   include/linux/trace_events.h |  3 +++
>>   kernel/trace/trace.c         | 11 +++++++++++
>>   kernel/trace/trace_events.c  | 22 ++++++++++++++++++++++
>>   3 files changed, 36 insertions(+)
>>
>> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
>> index d7b7d85..0cc99a8 100644
>> --- a/include/linux/trace_events.h
>> +++ b/include/linux/trace_events.h
>> @@ -545,7 +545,10 @@ int trace_array_printk(struct trace_array *tr, unsigned long ip,
>>   struct trace_array *trace_array_create(const char *name);
>>   int trace_array_destroy(struct trace_array *tr);
>>   int ftrace_set_clr_event(struct trace_array *tr, char *buf, int set);
>> +struct trace_array *trace_array_lookup(const char *name);
>>   int trace_set_clr_event(const char *system, const char *event, int set);
>> +int trace_array_set_clr_event(struct trace_array *tr, const char *system,
>> +		const char *event, int set);
>>   
>>   /*
>>    * The double __builtin_constant_p is because gcc will give us an error
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index a60dc13..1d171fd 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -8465,6 +8465,17 @@ static int instance_rmdir(const char *name)
>>   	return ret;
>>   }
>>   
>> +struct trace_array *trace_array_lookup(const char *name)
>> +{
>> +	struct trace_array *tr;
>> +	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> Accessing the ftrace_trace_arrays requires taking the trace_types_lock.
> It should also probably increment the ref counter too, and then
> trace_array_put() needs to be called.
>
> This prevents the trace array from being freed while something has
> access to it.
>
> -- Steve

Agree - Noted!

Also, adding a similar change for trace_array_create which also returns 
a ptr to a newly created trace_array so will face the same issue.

Since trace_array_lookup and trace_array_create will be accompanied by a 
trace_array_destroy once the use of the trace_array is done, 
decrementing the reference ctr here.

Sending a v2 to address this.


Thanks,

Divya

>
>> +		if (tr->name && strcmp(tr->name, name) == 0)
>> +			return tr;
>> +	}
>> +	return NULL;
>> +}
>> +EXPORT_SYMBOL_GPL(trace_array_lookup);
>> +
>>   static __init void create_trace_instances(struct dentry *d_tracer)
>>   {
>>   	trace_instance_dir = tracefs_create_instance_dir("instances", d_tracer,
>> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
>> index 445b059..c126d2c 100644
>> --- a/kernel/trace/trace_events.c
>> +++ b/kernel/trace/trace_events.c
>> @@ -859,6 +859,28 @@ int trace_set_clr_event(const char *system, const char *event, int set)
>>   }
>>   EXPORT_SYMBOL_GPL(trace_set_clr_event);
>>   
>> +/**
>> + * trace_array_set_clr_event - enable or disable an event for a trace array
>> + * @system: system name to match (NULL for any system)
>> + * @event: event name to match (NULL for all events, within system)
>> + * @set: 1 to enable, 0 to disable
>> + *
>> + * This is a way for other parts of the kernel to enable or disable
>> + * event recording to instances.
>> + *
>> + * Returns 0 on success, -EINVAL if the parameters do not match any
>> + * registered events.
>> + */
>> +int trace_array_set_clr_event(struct trace_array *tr, const char *system,
>> +		const char *event, int set)
>> +{
>> +	if (!tr)
>> +		return -ENODEV;
>> +
>> +	return __ftrace_set_clr_event(tr, NULL, system, event, set);
>> +}
>> +EXPORT_SYMBOL_GPL(trace_array_set_clr_event);
>> +
>>   /* 128 should be much more than enough */
>>   #define EVENT_BUF_SIZE		127
>>