io_uring/io-wq: re-check IO_WQ_BIT_EXIT for each linked work item

io_uring/io-wq: re-check IO_WQ_BIT_EXIT for each linked work item

commit 10dc95939817 ("io_uring/io-wq: check IO_WQ_BIT_EXIT inside work
run loop") fixed the obvious case where io_worker_handle_work() took one
exit-bit snapshot before draining pending work, but the fix stops one
level too early.

io_worker_handle_work() now re-checks IO_WQ_BIT_EXIT in its outer work
run loop, yet it still snapshots that bit once before processing a whole
dependent linked-work chain. If io_wq_exit_start() sets IO_WQ_BIT_EXIT
after the first linked item has started, the remaining linked items can
still reuse stale do_kill = false, skip IO_WQ_WORK_CANCEL, and continue
running after exit has begun.

Move the check further inside, so it covers linked items too. Note: this
is a syzbot special as it loves setting up tons of slow linked work on
weird devices like msr that take forever to read, and immediately close
the ring. Exit then takes a long time.

Fixes: 10dc95939817 ("io_uring/io-wq: check IO_WQ_BIT_EXIT inside work run loop")
Cc: stable@vger.kernel.org
Signed-off-by: Runyu Xiao <runyu.xiao@seu.edu.cn>
Link: https://patch.msgid.link/20260527172203.2043962-1-runyu.xiao@seu.edu.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>