linux-kernel - Re: [PATCH 1/3] uprobes/core: Remove redundant lock_page/unlock

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Date:	Thu, 31 May 2012 18:56:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [PATCH 1/3] uprobes/core: Remove redundant lock_page/unlock_page

On Thu, 2012-05-31 at 20:37 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2012-05-31 13:58:38]:
> 
> > On Thu, 2012-05-31 at 17:16 +0530, Srikar Dronamraju wrote:
> > > From: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> > > 
> > > No need to lock the page when copying the opcode in read_opcode().
> > 
> > It would be good if the changelog said _why_ this is so :-)
> 
> In read_opcode(), we have the reference for the page and we only are reading
> from the the page. i.e we are neither modifying the page contents, not
> the page attributes. 

Fair enough, so put that in the changelog. The changelog should explain
things, not raise questions.

> Existing kernel code has enough examples where we read the contents
> of the page without taking the page lock.

Yes, but that doesn't tell us this site is ok, doing it because others
do isn't an argument.

> Further this was discussed here too https://lkml.org/lkml/2012/4/17/361.

That wasn't a discussion, that was two people saying they don't know.
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