[PATCH bpf-next v2 3/3] sched_ext: Stop citing the x86 JIT for the cmask cmpxchg loop

[Puranjay Mohan]

cid.bpf.h explains its hand-rolled cmpxchg loop by saying the x86 BPF JIT
rejects BPF_OR | BPF_FETCH and BPF_AND | BPF_FETCH on arena pointers, and
names bpf_jit_supports_insn() as the place that does it. That function no
longer rejects anything, so the comment sends the reader to code that
contradicts it.

The loop itself still has to stay: arm64 without LSE rejects every arena
read-modify-write atomic. Reword the comment to say that instead.

Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
---
 tools/sched_ext/include/scx/cid.bpf.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/sched_ext/include/scx/cid.bpf.h b/tools/sched_ext/include/scx/cid.bpf.h
index 69fb4e97bc771..41060dcf5f5f2 100644
--- a/tools/sched_ext/include/scx/cid.bpf.h
+++ b/tools/sched_ext/include/scx/cid.bpf.h
@@ -130,10 +130,10 @@ static __always_inline bool cmask_test(u32 cid, const struct scx_cmask __arena *
 }
 
 /*
- * x86 BPF JIT rejects BPF_OR | BPF_FETCH and BPF_AND | BPF_FETCH on arena
- * pointers (see bpf_jit_supports_insn() in arch/x86/net/bpf_jit_comp.c). Only
- * BPF_CMPXCHG / BPF_XCHG / BPF_ADD with FETCH are allowed. Implement
- * test_and_{set,clear} and the atomic set/clear via a cmpxchg loop.
+ * Not every BPF JIT accepts BPF_OR | BPF_FETCH and BPF_AND | BPF_FETCH on
+ * arena pointers: arm64 without LSE rejects every arena read-modify-write
+ * atomic. Implement test_and_{set,clear} and the atomic set/clear via a
+ * cmpxchg loop so this works everywhere.
  *
  * CMASK_CAS_TRIES is sized so exhausting it means seconds of real spinning
  * on one word - past any plausible contention. Abort hard.
-- 
2.53.0-Meta