io_uring/poll: cancel all matching requests in one pass

With IORING_ASYNC_CANCEL_ALL, __io_async_cancel() calls io_try_cancel()
until nothing is found, and io_poll_cancel() cancels one request per
call, starting each lookup at the beginning of the hash. A cancelled
request stays hashed until its task work runs, so every lookup walks
past the ones already cancelled. Cancelling K of N pending polls visits
about K*N/2 requests in one io_uring_enter(), with uring_lock held.

A single CANCEL_FD|ALL on 80000 polls on one eventfd takes 17.06s,
0.02s with this change. With voluntary preemption the soft lockup
watchdog fires:

  watchdog: BUG: soft lockup - CPU#1 stuck for 26s! [repro:100]
  RIP: 0010:io_cancel_req_match+0x19/0xa0
   io_poll_cancel+0x73/0x170
   io_try_cancel+0x7b/0xf0
   __io_async_cancel+0x41/0x110

Walk the hash once and cancel every match, as io_cancel_remove() does
for futex and waitid. Cancel by user_data only needs its own bucket.
__io_async_cancel() still loops until nothing is found, so a request
that io-wq parks in the poll hash meanwhile is still cancelled.

A cancel that matches one request now walks the whole hash twice instead
of stopping at the match.

Cc: stable@vger.kernel.org
Fixes: 8e29da69fead ("io_uring: add support for IORING_ASYNC_CANCEL_ALL")
Signed-off-by: Shang En Sim <sim@shangen.org>
Link: https://patch.msgid.link/20261030050319.1448530-3-sim@shangen.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions