[PATCH 7.2 16/88] io-wq: order the exit bit against worker creation task work

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

7.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christian Brauner <brauner@kernel.org>

Commit 0e5663856b69219ef2f02e04bac9f5c5be19718a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 io_uring/io-wq.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/io_uring/io-wq.c
+++ b/io_uring/io-wq.c
@@ -1319,6 +1319,8 @@ static bool io_task_work_match(struct ca
 void io_wq_exit_start(struct io_wq *wq)
 {
 	set_bit(IO_WQ_BIT_EXIT, &wq->state);
+	/* Pairs with task_work_add() in io_queue_worker_create(). */
+	smp_mb__after_atomic();
 }
 
 static void io_wq_cancel_tw_create(struct io_wq *wq)