Re: [PATCH v2 bpf 3/4] bpf: Read producer_pos before overwrite_pos in ringbuf_avail_data_sz()

[Israel Téllez]

Please drop this patch from the series; patches 1, 2 and 4 stand on their own.

Reading producer_pos first does follow the contract documented in
__bpf_ringbuf_reserve(), but it also allows overwrite_pos to be observed
ahead of the producer_pos snapshot, and then prod_pos - over_pos underflows
and min() falls back to prod_pos - cons_pos, which in overwrite mode is not
bounded by the ring size. That trades a bounded staleness for an unbounded
overestimate, which is worse than what it fixes.

Making the documented order safe needs an additional guard for over_pos
running ahead of the snapshot, so it belongs in its own patch rather than
here.