linux-kernel - Re: [PATCH] tracing/filters: allow on-the-fly filter switching

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Date:	Wed, 15 Apr 2009 18:21:53 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tom Zanussi <tzanussi@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] tracing/filters: allow on-the-fly filter switching

On Tue, Apr 14, 2009 at 11:32:25PM -0500, Tom Zanussi wrote:
> On Tue, 2009-04-14 at 22:56 +0200, Frederic Weisbecker wrote:
> > On Mon, Apr 13, 2009 at 03:17:50AM -0500, Tom Zanussi wrote:
> 
> [...]
> 
> > 
> > 
> > BTW, this issue might be already present before this patch.
> > What happens if:
> > 
> > 
> > 	T1				T2
> > 
> > event_filter_read() {
> >   filter_print_preds() {
> >     for (i = 0; i < n_preds; i++) {
> > 	pred = preds[i];
> > 				    event_filter_write() {
> > 					filter_disable_preds();
> > 					filter_clear_preds() {
> > 					  kfree(pred->field_name);
> > 	field_name = pred->field_name;
> > 	// CRASH!!!
> > 
> > 
> > You need a mutex to protect these two callbacks.
> > It would also protect concurrent calls to event_filter_write(),
> > which would result in random.
> > 
> 
> Yeah, Li Zefan had already fixed this, but it wasn't included in this
> patch.  Looks like he'll be resubmitting that part...
> 
> > 
> > 
> 
> [...]
> 
> > Also, is filter_add_pred() supposed to be available for in-kernel
> > uses by other tracers or something?
> 
> No, the current callers were the only ones I'd planned on (it's not
> static because code in trace_events.c needs to call it).  But, do you
> see a use for it by other tracers?



May be in the future, it's possible that a tracer might want to
set filters by itself.
But I don't think it has to be fixed now because there are nothing
like that for now.

So, no problem :-)

Frederic.

 
> > If this is planned, the locking could be even deeper than my comments.
> > 
> > Other than these comments:
> > 
> > Acked-by: Frederic Weisbecker <fweisbec@...il.com>
> > 
> > Thanks!
> > 
> 
> Thanks for reviewing it!
> 
> Tom
> 

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