From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1755942AbZETUMb@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755942AbZETUMb (ORCPT <rfc822;w@1wt.eu>);
	Wed, 20 May 2009 16:12:31 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754291AbZETUMY
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Wed, 20 May 2009 16:12:24 -0400
Received: from mx1.redhat.com ([66.187.233.31]:43749 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753579AbZETUMY (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 20 May 2009 16:12:24 -0400
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
From: Roland McGrath <roland@redhat.com>
To: Oleg Nesterov <oleg@redhat.com>
X-Fcc: ~/Mail/linus
Cc: Ingo Molnar <mingo@elte.hu>, Christoph Hellwig <hch@infradead.org>,
       Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>,
       Vitaly Mayatskikh <v.mayatskih@gmail.com>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Q: put_user_try & co (Was: [PATCH 1/5] Split wait_noreap_copyout())
In-Reply-To: Oleg Nesterov's message of  Wednesday, 20 May 2009 21:03:12 +0200 <20090520190312.GA32333@redhat.com>
References: <1242036759-4025-1-git-send-email-v.mayatskih@gmail.com>
	<1242036759-4025-2-git-send-email-v.mayatskih@gmail.com>
	<20090511120418.GA3859@infradead.org>
	<20090511121708.GD13954@elte.hu>
	<20090520190312.GA32333@redhat.com>
Emacs: Lovecraft was an optimist.
Message-Id: <20090520201127.48B54FC38D@magilla.sf.frob.com>
Date: Wed, 20 May 2009 13:11:27 -0700 (PDT)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

> 	#define __put_user_asm_ex(...)				\
> 		asm volatile(					\
> 			"1:	mov ..."			\
> 			_ASM_EXTABLE(1b, &__efault_label)	\
> 		     	: : ...)

You mean &&__efault_label here (it's a funny syntax, but that's how it is).
&&label is a GCC extension that I'm not sure the kernel has used before.

I think it can be touchy to have an asm jump into compiled code that way.
e.g., perhaps the compiler produced:

	mov reg, 40(sp)
	mov $123, reg
	#APP
	... inside of your asm ...
	#NO_APP
	mov 40(sp), reg

or some such thing.  If you jump away from inside the asm, you won't ever
do "mov 40(sp), reg".  But the compiler might think that reg has its
original value at the __efault_label: code location.

Perhaps more important than any particular compiler-confusion scenario we
can come up with is simply that this would be an obscure corner of code
generation in the compiler that the kernel has not evoked before.  There
might be bugs or oddities in various compilers of various vintages, that
we don't know about because they never came up before.


Thanks,
Roland