linux-kernel - Re: PATCH? trace_remove_event_call() should fail if call is active

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Date:	Wed, 03 Jul 2013 12:12:36 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: PATCH? trace_remove_event_call() should fail if call is active

(2013/07/03 11:57), Steven Rostedt wrote:
> On Wed, 2013-07-03 at 11:42 +0900, Masami Hiramatsu wrote:
>> (2013/07/03 7:23), Oleg Nesterov wrote:
>>> On 07/02, Oleg Nesterov wrote:
>>>>
>>>> So please ignore modules ;)
>>>
>>> Or lets discuss the change above.
>>
>> No, I think this still doesn't ensure that we can remove dynamic
>> event safely. Since the event is related to several files under
>> events/ dir and buffer instances, someone can just stay open the
>> files while the event is removed and read/write it.
>> Perhaps, we need per-event_call refcounter not per-trace_array
>> one, and do as below.
> 
> Or both. I need the trace-array counter for rmdir, and don't want to
> check every event. But that doesn't mean we can't have a event counter.

Agreed.

>> Open file:
>>  -> lock event_mutex
>>  -> find event_call and event_file
>>  -> get event_call
>>  -> unlock event_mutex
>>
>> Close:
>>  -> lock event_mutex
>>  -> put event_call
>>  -> unlock event_mutex
>>
>> Remove event (via kprobe_events):
>>  -> lock event_mutex
>>  -> find event_call
>>  -> -EBUSY if event is enabled or refcount != 0
>>     (here, no one accessing the event and not enabled)
>>  -> unregister_kprobe
>>  -> remove event
>>  -> unlock event_mutex
>>
>> The key is holding event_mutex *and* getting refcount
>> under any operation.
> 
> And this would be to the event call, not the event file itself, right?

Right, because an event file just indicates an instance of the
event. This "Remove event" totally removes the event including
all instances.

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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