Hi Daniel, On Wed, Nov 11, 2015 at 11:42:11AM +0100, Daniel Borkmann wrote: > 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.starovoi...@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.
Yikes, so the memory-model for BPF is based around the deprecated GCC __sync builtins, that inherit their semantics from ia64? Any reason not to use the C11-compatible __atomic builtins[1] as a base? > 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. Ok, so it does sound like eBPF needs to define/choose a memory-model and I worry that riding on the back of __sync isn't necessarily the right thing to do, particularly as its fallen out of favour with the compiler folks. On weakly-ordered architectures, it's also going to result in heavy-weight barriers for all atomic operations. Will [1] https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
