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

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Message-ID: <1372820252.22688.94.camel@gandalf.local.home>
Date:	Tue, 02 Jul 2013 22:57:32 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
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

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.


> 
> 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?

-- Steve

> And of course, we can unregister the kprobe outside of
> the event_mutex, but it still need a synchronize_sched
> for safety.
> 
>  -> lock event_mutex
>  -> wait_for_rcu (to ensure no disabled kprobe accesses the event)
>  -> find event_call
>  -> -EBUSY if event is enabled or refcount != 0
>     (here, no one accessing the event and not enabled)
>  -> remove event
>  -> unlock event_mutex
>  -> unregister_kprobe
> 
> Thank you,
> 
> 


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