[PATCH] ARM: add cmpxchg64 helper for ARMv7-M

[Russell King - ARM Linux]

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.

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