From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pablo Neira Ayuso Subject: Re: [PATCH] ARM: add cmpxchg64 helper for ARMv7-M Date: Sat, 10 Dec 2016 13:32:34 +0100 Message-ID: <20161210123234.GA5468@salvia> References: <20161210103646.1407256-1-arnd@arndb.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Russell King , netdev@vger.kernel.org, "Paul E. McKenney" , "David S. Miller" , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org To: Arnd Bergmann Return-path: Content-Disposition: inline In-Reply-To: <20161210103646.1407256-1-arnd@arndb.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Hi Arnd, On Sat, Dec 10, 2016 at 11:36:34AM +0100, Arnd Bergmann wrote: > A change to the netfilter code in net-next introduced the first caller of > cmpxchg64 that can get built on ARMv7-M, leading to an error from the > assembler that points out the lack of 64-bit atomics on this architecture: > > /tmp/ccMe7djj.s: Assembler messages: > /tmp/ccMe7djj.s:367: Error: selected processor does not support `ldrexd r0,r1,[lr]' in Thumb mode > /tmp/ccMe7djj.s:371: Error: selected processor does not support `strexd ip,r2,r3,[lr]' in Thumb mode > /tmp/ccMe7djj.s:389: Error: selected processor does not support `ldrexd r8,r9,[r7]' in Thumb mode > /tmp/ccMe7djj.s:393: Error: selected processor does not support `strexd lr,r0,r1,[r7]' in Thumb mode > scripts/Makefile.build:299: recipe for target 'net/netfilter/nft_counter.o' failed > > This makes ARMv7-M use the same emulation from asm-generic/cmpxchg-local.h > that we use on architectures earlier than ARMv6K, to fix the build. The > 32-bit atomics are available on ARMv7-M and we keep using them there. > This ARM specific change is probably something we should do regardless > of the netfilter code. > > However, looking at the new nft_counter_reset() function in nft_counter.c, > this looks incorrect to me not just on ARMv7-M but also on other > architectures, with at least the following possible race: Right, Eric Dumazet already spotted this problem. I'm preparing a patch that doesn't require cmpxchg64(). Will keep you on Cc. Thanks.