linux-kernel - Re: [RFC][PATCH 4/5] tracing/filters: Provide basic regex support

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Date:	Fri, 7 Aug 2009 07:19:31 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tom Zanussi <tzanussi@...il.com>
Cc:	Li Zefan <lizf@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC][PATCH 4/5] tracing/filters: Provide basic regex support

On Thu, Aug 06, 2009 at 11:14:57PM -0500, Tom Zanussi wrote:
> Hi Frederic,
> 
> On Thu, 2009-08-06 at 03:49 +0200, Frederic Weisbecker wrote:
> > On Thu, Aug 06, 2009 at 09:14:27AM +0800, Li Zefan wrote:
> > > Frederic Weisbecker wrote:
> > > > On Mon, Aug 03, 2009 at 01:39:58PM +0800, Li Zefan wrote:
> > > >> Frederic Weisbecker wrote:
> > > >>> This patch provides basic support for regular expressions in filters.
> > > >>> The common filter file doesn't support any regex but a new
> > > >>> filter_regex file is created for each subsystem/event.
> > > >>>
> > > >>> It supports the following types of regexp:
> > > >>>
> > > >>> - *match_beginning
> > > >>> - *match_middle*
> > > >>> - match_end*
> > > >>> - !don't match
> > > >>>
> > > >> I don't see why adding "filter_regex" is necessary, why not just make
> > > >> "filter" support regex?
> > > > 
> > > > I did that because I feared about people beeing unable to filter using
> > > > * as a real character inside a filter string.
> > > > If we are using only one file, we are forced to get the '*' interpreted.
> > > > That would also break the ABI.
> > > > 
> > > 
> > > I think we don't maintain stable ABI for debugfs and we've been changing
> > > the "ABI" in debugfs/tracing/, but of course we shouldn't change it
> > > without good reasons.
> > 
> > 
> > Yeah. May be I'm too much careful there but i prefer not to take the risk.
> > 
> >  
> > > One alternative is to use '"' to prevent '*' to be translated, but seems
> > > using "filter_regex" is more convenient, so I agree with this patch.
> > 
> > 
> > The problem is that most of the time, the '"' is about mandatory to delimit
> > a string.
> > Say you want to filter the lock name &REISERFS_SB(sb)->lock, you can't do that
> > by just typing:
> > 
> > echo name == &REISERFS_SB(sb)->lock > events/lockdep/filter
> > 
> > because the '&' character is considered as an operator and that will trigger
> > an error.
> > Instead you must type:
> > 
> > echo 'name == "&REISERFS_SB(sb)->lock"' > events/lockdep/filter
> > 
> > The '' are there to delimit the string for echo, and the "" are interpreted
> > by the filter api which take it as a whole string and not strings interleaved
> > with operators. I even fear - and > may be considered as operators if we omit
> > the "".
> > 
> > I also thought about using an antislash for those who don't want the * to
> > be interpreted. But although it seems intuitive, it's not as much as two
> > distinct and visible files.
> > 
> > Also, we can think about the fact the regexp support could be extended one
> > day if someone needs to. And we could then encounter even more ambiguous
> > characters such as $, ^, [, {, etc...
> > 
> 
> This a nice new feature - I considered doing it (not complete regexp
> support, just * in strings) for the original patch, but ran out of time
> - glad you added it.
> 
> I still think it makes sense to have some basic support for * in the
> regular filter file, so I'd vote for getting rid of the filter_regex
> file for now and just adding * support with the antislash escape to the
> regular filter file.  If you later wanted to add more full-fledged
> regexp support and it didn't make sense to do it in the regular filter
> file, then you could go crazy and add the filter_regex later...


Ok, if you and Li both prefer the single file, I'm fine with that.
It shouldn't really harm.
I'll change that in the next version.

Thanks.

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