Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755685Ab2BBLLM (ORCPT ); Thu, 2 Feb 2012 06:11:12 -0500 Received: from mail-qy0-f174.google.com ([209.85.216.174]:40862 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751481Ab2BBLLJ convert rfc822-to-8bit (ORCPT ); Thu, 2 Feb 2012 06:11:09 -0500 MIME-Version: 1.0 In-Reply-To: <20120201151918.GC16714@quack.suse.cz> References: <20120201151918.GC16714@quack.suse.cz> Date: Thu, 2 Feb 2012 11:11:09 +0000 Message-ID: Subject: Re: Memory corruption due to word sharing From: James Courtier-Dutton To: Jan Kara Cc: LKML , linux-ia64@vger.kernel.org, Linus Torvalds , dsterba@suse.cz, ptesarik@suse.cz, rguenther@suse.de, gcc@gcc.gnu.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2661 Lines: 55 On 1 February 2012 15:19, Jan Kara wrote: >  Hello, > >  we've spotted the following mismatch between what kernel folks expect > from a compiler and what GCC really does, resulting in memory corruption on > some architectures. Consider the following structure: > struct x { >    long a; >    unsigned int b1; >    unsigned int b2:1; > }; > > We have two processes P1 and P2 where P1 updates field b1 and P2 updates > bitfield b2. The code GCC generates for b2 = 1 e.g. on ia64 is: >   0:   09 00 21 40 00 21       [MMI]       adds r32=8,r32 >   6:   00 00 00 02 00 e0                   nop.m 0x0 >   c:   11 00 00 90                         mov r15=1;; >  10:   0b 70 00 40 18 10       [MMI]       ld8 r14=[r32];; >  16:   00 00 00 02 00 c0                   nop.m 0x0 >  1c:   f1 70 c0 47                         dep r14=r15,r14,32,1;; >  20:   11 00 38 40 98 11       [MIB]       st8 [r32]=r14 >  26:   00 00 00 02 00 80                   nop.i 0x0 >  2c:   08 00 84 00                         br.ret.sptk.many b0;; > > Note that gcc used 64-bit read-modify-write cycle to update b2. Thus if P1 > races with P2, update of b1 can get lost. BTW: I've just checked on x86_64 > and there GCC uses 8-bit bitop to modify the bitfield. > > We actually spotted this race in practice in btrfs on structure > fs/btrfs/ctree.h:struct btrfs_block_rsv where spinlock content got > corrupted due to update of following bitfield and there seem to be other > places in kernel where this could happen. > > I've raised the issue with our GCC guys and they said to me that: "C does > not provide such guarantee, nor can you reliably lock different > structure fields with different locks if they share naturally aligned > word-size memory regions.  The C++11 memory model would guarantee this, > but that's not implemented nor do you build the kernel with a C++11 > compiler." > > So it seems what C/GCC promises does not quite match with what kernel > expects. I'm not really an expert in this area so I wanted to report it > here so that more knowledgeable people can decide how to solve the issue... > >                                                                Honza > -- > Jan Kara > SUSE Labs, CR What is the recommended work around for this problem? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/