linux-kernel - Re: [PATCH] tracing/lockdep: turn lock->name into an array

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Message-ID: <20090413225846.GA817@elte.hu>
Date:	Tue, 14 Apr 2009 00:58:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Zhaolei <zhaolei@...fujitsu.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] tracing/lockdep: turn lock->name into an array


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Tue, Apr 14, 2009 at 12:36:06AM +0200, Frederic Weisbecker wrote:
> > Impact: allow filtering by lock name / fix module tracing
> > 
> > Currently, the "lock acquired" event is traced using a TRACE_EVENT.
> > But we can't use the char * type for the name without risking to
> > dereference a freed pointer. A lock name can come from a module
> > towards lockdep and it is risky to only store its address because we
> > defer its name printing.
> > 
> > That's why this patch uses a fixed array size and copy the name.
> > Also it lets us filter the lock name because the event filtering
> > doesn't handle the char pointers. Such support is not needed yet since
> > the events don't use it for now because it is rarely easy to keep track
> > of a string while we defer its output.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > ---
> >  include/trace/lockdep_event_types.h |   13 +++++++++++--
> >  1 files changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/trace/lockdep_event_types.h b/include/trace/lockdep_event_types.h
> > index 863f1e4..68f84f4 100644
> > --- a/include/trace/lockdep_event_types.h
> > +++ b/include/trace/lockdep_event_types.h
> > @@ -32,18 +32,27 @@ TRACE_FORMAT(lock_contended,
> >  	TP_FMT("%s", lock->name)
> >  	);
> >  
> > +#define LOCK_NAME_SIZE	25
> 
> 
> 
> This constant may look a bit weird.
> I just started with the assumption that a full lock name
> will rarely exceed this length.
> 
> If you agree with it, I will expand the conversion of lockdep
> TRACE_FORMAT to TRACE_EVENTS with the same assumption.
> So that we will be able to use filters with locks events.

Sure.

But 25 might be on the narrow side. The names come from macros and 
we do have a few cases of particularly long sequences of: 
spin_lock(&my_subsys->my_bus->my_driver->my_hw->my_object->my_lock) 
easily spanning 25 characters. And stripping will strip away the 
most useful (final) bits of the word.

	Ingo
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