From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: [PATCH] ARM: add cmpxchg64 helper for ARMv7-M Date: Tue, 13 Dec 2016 16:58:12 +0000 Message-ID: <20161213165811.GZ14217@n2100.armlinux.org.uk> References: <20161210103646.1407256-1-arnd@arndb.de> <20161210123234.GA5468@salvia> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Arnd Bergmann , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, coreteam@netfilter.org, netfilter-devel@vger.kernel.org, "Paul E. McKenney" , "David S. Miller" , linux-arm-kernel@lists.infradead.org To: Pablo Neira Ayuso Return-path: Content-Disposition: inline In-Reply-To: <20161210123234.GA5468@salvia> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org List-Id: netfilter-devel.vger.kernel.org On Sat, Dec 10, 2016 at 01:32:34PM +0100, Pablo Neira Ayuso wrote: > 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. Please keep me on the Cc as well so I know what's happening, thanks. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.