io_uring/io-wq: don't lose the signal that ends work

In the Linux kernel, the following vulnerability has been resolved:

io_uring/io-wq: don't lose the signal that ends work

When a thread exits, only its tracked requests are cancelled. Then its
io-wq is torn down. io_wq_exit_workers() signals every worker, to get it
out of an interruptible sleep, and waits for all of them to exit. Before
they do, they go through the work that is left. Unbounded work is
cancelled. Bounded work is run, as it is expected to end by itself.

Not all of it does. An open has no file yet, so it counts as bounded, and
the open of a FIFO waits for the other end. That is what the signal is for.
But the workers are signalled once, and a worker that hasn't started the
work by then loses the signal:

io_wq_exit_workers() io_worker_handle_work()
io_wq_worker_wake()
set TIF_NOTIFY_SIGNAL
io_assign_current_work()
io_run_task_work()
clear TIF_NOTIFY_SIGNAL
io_wq_submit_work()
fifo_open()
wait_for_partner()
wait_for_completion_timeout()

The thread never gets out of do_exit():

IORING_OP_OPENAT of a FIFO without a writer, IOSQE_ASYNC
exit of the thread that has submitted it

hangs in more than 9 of 10 attempts (4 CPUs, KVM), until the process is
killed.

Set TIF_NOTIFY_SIGNAL for bounded work that is started with IO_WQ_BIT_EXIT
set. The bit is set before the workers are signalled, and it is tested
after the flag has been cleared, with clear_notify_signal() providing the
barrier. So either the flag stays, or the bit is seen.

Work that is started while the io-wq exits is treated like work that is
being run at that time: it isn't interrupted unless it waits
interruptibly.