On Wed, Nov 11, 2015 at 01:21:04PM +0100, Daniel Borkmann wrote: > On 11/11/2015 12:58 PM, Will Deacon wrote: > >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? > > Hmm, gcc doesn't have an eBPF compiler backend, so this won't work on > gcc at all. The eBPF backend in LLVM recognizes the __sync_fetch_and_add() > keyword and maps that to a BPF_XADD version (BPF_W or BPF_DW). In the > interpreter (__bpf_prog_run()), as Eric mentioned, this maps to atomic_add() > and atomic64_add(), respectively. So the struct bpf_insn prog[] you saw > from sock_example.c can be regarded as one possible equivalent program > section output from the compiler.
Ok, so if I understand you correctly, then __sync_fetch_and_add() has different semantics depending on the backend target. That seems counter to the LLVM atomics Documentation: http://llvm.org/docs/Atomics.html which specifically calls out the __sync_* primitives as being sequentially-consistent and requiring barriers on ARM (which isn't the case for atomic[64]_add in the kernel). If we re-use the __sync_* naming scheme in the source language, I don't think we can overlay our own semantics in the backend. The __sync_fetch_and_add primitive is also expected to return the old value, which doesn't appear to be the case for BPF_XADD. Will -- 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
