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.