linux-kernel - Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

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Date:	Wed, 30 Nov 2011 13:25:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Wilson <wilsons@...rt.ca>, tulasidhard@...il.com
Subject: Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

On Tue, 2011-11-29 at 21:52 +0530, Srikar Dronamraju wrote:
> The rules that I am using are: 
> 
> mmap_uprobe() increments the count if 
>         - it successfully adds a breakpoint.
>         - it not add a breakpoint, but sees that there is a underlying
>           breakpoint (via a read_opcode call).
> 
> munmap_uprobe() decrements the count if 
>         - it sees a underlying breakpoint,  (via  a read_opcode call)
>         - Subsequent unregister_uprobe wouldnt find the breakpoint
>           unless a mmap_uprobe kicks in, since the old vma would be
>           dropped just after munmap_uprobe.
> 
> register_uprobe increments the count if:
>         - it successfully adds a breakpoint.
> 
> unregister_uprobe decrements the count if:
>         - it sees a underlying breakpoint and removes successfully. 
>                         (via a read_opcode call)
>         - Subsequent munmap_uprobe wouldnt find the breakpoint
>           since there is no underlying breakpoint after the
>           breakpoint removal. 

The problem I'm having is that such stuff isn't included in the patch
set.

We've got both comments in the C language and Changelog in our patch
system, yet you consistently fail to use either to convey useful
information on non-trivial bits like this.

This leaves the reviewer wondering if you've actually considered stuff
properly, then me actually finding bugs in there does of course
undermine that even further.

What I really would like is for this patch set not to have such subtle
stuff at all, esp. at first. Once its in and its been used a bit we can
start optimizing and add subtle crap like this.


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