linux-kernel - Re: [PATCHv9 2.6.35-rc4-tip 2/13] uprobes: Breakpoint insertion/removal in user space applications.

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Message-ID: <20100804124822.GC28212@linux.vnet.ibm.com>
Date:	Wed, 4 Aug 2010 18:18:22 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Hellwig <hch@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Randy Dunlap <rdunlap@...otime.net>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Mark Wielaard <mjw@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Naren A Devaiah <naren.devaiah@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv9 2.6.35-rc4-tip 2/13]  uprobes: Breakpoint
 insertion/removal in user space applications.

* Peter Zijlstra <peterz@...radead.org> [2010-08-04 14:05:28]:

> On Tue, 2010-07-20 at 12:52 +0530, Srikar Dronamraju wrote:
> > > Just wondering why these are function pointers.  Do we exepect an
> > > architecture to provide different versions of these for say 32 vs 64-bit
> > > binaries?  If not just making these arch provided helpers might be a lot
> > > simpler.  Especially in the current version where only very few of these
> > > are overriden by the architecture at all.
> > > 
> > 
> > Some of these functions are purely optional example being
> > validate_address. 
> > 
> > Some of these functions need not be defined by the architecture in
> > which case we default to the functions defined in common code. 
> > examples being: read_opcode, set_bkpt, and set_orig_insn.
> > 
> > Some of these functions are architecture mode specific, for example 
> > there is a architecture specific pre_xol needed for x86_64. However
> > generic pre_xol for x86_32 would suffice for x86_32.
> > 
> > Some of these functions need to be mandatorily defined by the
> > architecture. example being set_ip and analyze_insn.
> > 
> > Apart from the above flexibilities and enforcements that we can make
> > when we use function pointers, its would be handy to incorporate
> > more enhancements like return probes and booster. 
> 
> Still not sure why you're using this vector though, why not use weak
> function for optionals and defaults and no implementation for mandatory
> functions (and if the implementations fails to provide it, that will
> result in a link error).

Yes, we can certainly use weak functions instead of pointers. 
One another reason why we had these as function pointers in a
structure was that it would easy be  for a person porting uprobes to a
new architecture. i.e person porting to a new architecture knows in one
place(structure) which all functions need to be provided. 

However I would go with your suggestion and make the changes to use weak
functions in the next version of the patchset. 

> 
> Are there likely to be multiple different versions of this method vector
> around on a running kernel?

No for a running kernel, there will be only one method vector.

Also wanted to check with if you had tried perf probes and had
comments/suggestions on any of the other patches in the patchset.

--
Thanks and Regards
Srikar
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