Messages in this thread | | | Date | Wed, 11 Nov 2015 11:42:11 +0100 | From | Daniel Borkmann <> | Subject | Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction |
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On 11/11/2015 11:24 AM, Will Deacon wrote: > On Wed, Nov 11, 2015 at 09:49:48AM +0100, Arnd Bergmann wrote: >> On Tuesday 10 November 2015 18:52:45 Z Lim wrote: >>> On Tue, Nov 10, 2015 at 4:42 PM, Alexei Starovoitov >>> <alexei.starovoitov@gmail.com> wrote: >>>> On Tue, Nov 10, 2015 at 04:26:02PM -0800, Shi, Yang wrote: >>>>> On 11/10/2015 4:08 PM, Eric Dumazet wrote: >>>>>> On Tue, 2015-11-10 at 14:41 -0800, Yang Shi wrote: >>>>>>> aarch64 doesn't have native support for XADD instruction, implement it by >>>>>>> the below instruction sequence: >>> >>> aarch64 supports atomic add in ARMv8.1. >>> For ARMv8(.0), please consider using LDXR/STXR sequence. >> >> Is it worth optimizing for the 8.1 case? It would add a bit of complexity >> to make the code depend on the CPU feature, but it's certainly doable. > > What's the atomicity required for? Put another way, what are we racing > with (I thought bpf was single-threaded)? Do we need to worry about > memory barriers? > > Apologies if these are stupid questions, but all I could find was > samples/bpf/sock_example.c and it didn't help much :(
The equivalent code more readable in restricted C syntax (that can be compiled by llvm) can be found in samples/bpf/sockex1_kern.c. So the built-in __sync_fetch_and_add() will be translated into a BPF_XADD insn variant.
What you can race against is that an eBPF map can be _shared_ by multiple eBPF programs that are attached somewhere in the system, and they could all update a particular entry/counter from the map at the same time.
Best, Daniel
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