Three faults on the Windows pad path, two of them races that only bite when a
game drives a pad hard enough for two callbacks to overlap.
pf-gamepad's output ring could hand the host a torn report. Publishing is a
read-modify-write — read the cursor, write the slot it names, advance it — and
the framework dispatches output callbacks in parallel, so two could be inside
it at once: both read the same head, both wrote the SAME slot, and both stored
head+1, so the cursor moved once for two reports and the host read a single
entry with two reports mixed into it. An atomic fetch_add does not fix this. It
hands each writer its own slot but advances the cursor before the bytes exist,
so the host is then invited to read a slot still being filled. Serializing the
publish is what makes the cursor bump mean "the slot below is complete". The
ring exists to stop a rumble STOP being coalesced away, and a torn slot can eat
that STOP with no idle watchdog behind it.
Both drivers also promised the host an ordering they never established. The
host loads out_seq and rumble_seq with Acquire and says so in its own comments
— "Acquire pairs with the driver's publish-then-bump store order" — but the
drivers bumped both with plain writes, and an Acquire load pairs with a Release
store and nothing else. On a weakly-ordered core the host could see a fresh seq
against stale bytes. pf-xusb's rumble seq was racy in the same way as the ring:
two SET_STATE calls could both read one value and both write back value+1, so
the host saw one bump for two writes and skipped a level. A skipped stop is the
one that hurts — the pad buzzes until the ~2.5 s idle force-off notices the
game went quiet, which is what bounds the damage.
Diagnosing an unattached driver stalled the session. The pad service thread —
the one feeding input and rumble — waited up to two seconds for a pnputil
enumeration, per unattached pad, at exactly the moment a session was already
going wrong. The diagnosis now runs on its own thread. Off the hot path the
wait no longer has to be a compromise, so it is generous enough to report what
it actually found instead of giving up with "still enumerating" — which, given
pnputil routinely takes longer than the old budget, is what it usually did.
enricobuehler