io-wq: write next_work before dropping acct_lock

Commit 361aee4 ("io-wq: add intermediate work step between pending
list and active work") closed a race between a cancellation and the work
being removed from the wq for execution.  To ensure the request is
always reachable by the cancellation, we need to move it within the wq
lock, which also synchronizes the cancellation.  But commit
42abc95 ("io-wq: decouple work_list protection from the big
wqe->lock") replaced the wq lock here and accidentally reintroduced the
race by releasing the acct_lock too early.

In other words:

        worker                |     cancellation
work = io_get_next_work()     |
raw_spin_unlock(&acct->lock); |
			      |
                              | io_acct_cancel_pending_work
                              | io_wq_worker_cancel()
worker->next_work = work

Using acct_lock is still enough since we synchronize on it on
io_acct_cancel_pending_work.

Fixes: 42abc95 ("io-wq: decouple work_list protection from the big wqe->lock")
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20240416021054.3940-2-krisman@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: krisman

io_uring/io-wq.c

@@ -564,10 +564,7 @@ static void io_worker_handle_work(struct io_wq_acct *acct,
 		 * clear the stalled flag.
 		 */
 		work = io_get_next_work(acct, worker);
-		raw_spin_unlock(&acct->lock);
 		if (work) {
-			__io_worker_busy(wq, worker);
-
 			/*
 			 * Make sure cancelation can find this, even before
 			 * it becomes the active work. That avoids a window
@@ -578,9 +575,15 @@ static void io_worker_handle_work(struct io_wq_acct *acct,
 			raw_spin_lock(&worker->lock);
 			worker->next_work = work;
 			raw_spin_unlock(&worker->lock);
-		} else {
-			break;
 		}
+
+		raw_spin_unlock(&acct->lock);
+
+		if (!work)
+			break;
+
+		__io_worker_busy(wq, worker);
+
 		io_assign_current_work(worker, work);
 		__set_current_state(TASK_RUNNING);