io-wq: order the exit bit against worker creation task work

io_wq_exit_workers() cancels task work for queued worker creation with
task_work_cancel_match(). That has a plain load of task->task_works.

io_queue_worker_create() adds the new entry and tests IO_WQ_BIT_EXIT to
cancel it if the workqueue is already on its way out.

That must be ordered. task_work_add() has a full barrier via cmpxchg().
But io_wq_exit_start() sets the bit with set_bit() which doesn't have
any memory ordering.

So afaict, on weakly ordered architectures the exiting task may load
task_works before the store of the bit is visible. The other side tests
the bit before the store is visible as well. So the entry remains queued
with worker_refs and the exiting task waits on worker_done indefinitely.

If the task still has rings then io_uring_del_tctx_node() provides the
barrier via test_and_set_bit() in io_wq_set_exit_on_idle(). When it
has closed all rings though that barrier is gone.

Add the barrier after the set_bit().

Fixes: 71a85387546e ("io-wq: check for wq exit after adding new worker task_work")
Cc: stable@vger.kernel.org
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-4-8e4adb1619e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions