io_uring: ensure IORING_REGISTER_IOWQ_MAX_WORKERS works with SQPOLL

SQPOLL has a different thread doing submissions, we need to check for
that and use the right task context when updating the worker values.
Just hold the sqd->lock across the operation, this ensures that the
thread cannot go away while we poke at ->io_uring.

Link: https://github.com/axboe/liburing/issues/420
Fixes: 2e480058 ("io-wq: provide a way to limit max number of workers")
Reported-by: Johannes Lundberg <johalun0@gmail.com>
Tested-by: Johannes Lundberg <johalun0@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -10323,26 +10323,46 @@ static int io_unregister_iowq_aff(struct io_ring_ctx *ctx)
 static int io_register_iowq_max_workers(struct io_ring_ctx *ctx,
 					void __user *arg)
 {
-	struct io_uring_task *tctx = current->io_uring;
+	struct io_uring_task *tctx = NULL;
+	struct io_sq_data *sqd = NULL;
 	__u32 new_count[2];
 	int i, ret;
 
-	if (!tctx || !tctx->io_wq)
-		return -EINVAL;
 	if (copy_from_user(new_count, arg, sizeof(new_count)))
 		return -EFAULT;
 	for (i = 0; i < ARRAY_SIZE(new_count); i++)
 		if (new_count[i] > INT_MAX)
 			return -EINVAL;
 
+	if (ctx->flags & IORING_SETUP_SQPOLL) {
+		sqd = ctx->sq_data;
+		if (sqd) {
+			mutex_lock(&sqd->lock);
+			tctx = sqd->thread->io_uring;
+		}
+	} else {
+		tctx = current->io_uring;
+	}
+
+	ret = -EINVAL;
+	if (!tctx || !tctx->io_wq)
+		goto err;
+
 	ret = io_wq_max_workers(tctx->io_wq, new_count);
 	if (ret)
-		return ret;
+		goto err;
+
+	if (sqd)
+		mutex_unlock(&sqd->lock);
 
 	if (copy_to_user(arg, new_count, sizeof(new_count)))
 		return -EFAULT;
 
 	return 0;
+err:
+	if (sqd)
+		mutex_unlock(&sqd->lock);
+	return ret;
 }
 
 static bool io_register_op_must_quiesce(int op)