On Wed, Nov 11, 2015 at 12:38:31PM +0000, Will Deacon wrote: > > 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.
Yikes. That's double fail. Please don't do this. If you use the __sync stuff (and I agree with Will, you should not) it really _SHOULD_ be sequentially consistent, which means full barriers all over the place. And if you name something XADD (exchange and add, or fetch-add) then it had better return the previous value. atomic*_add() does neither. -- 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
