IB/mlx5: Resolve soft lock on massive reg MRs

commit 6bc1a656 upstream.

When calling reg_mr of large MRs (e.g. 4GB) from multiple processes
and MR caches can't supply the required amount of MRs the slow-path
of MR allocation may be used. In this case we need to serialize the
slow-path between the processes to avoid soft lock.

Fixes: e126ba97 ('mlx5: Add driver for Mellanox Connect-IB adapters')
Signed-off-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/infiniband/hw/mlx5/mlx5_ib.h

@@ -376,6 +376,8 @@ struct mlx5_ib_dev {
 	struct mlx5_ib_resources	devr;
 	struct mlx5_mr_cache		cache;
 	struct timer_list		delay_timer;
+	/* Prevents soft lock on massive reg MRs */
+	struct mutex			slow_path_mutex;
 	int				fill_delay;
 };
 

drivers/infiniband/hw/mlx5/mr.c

@@ -554,6 +554,7 @@ int mlx5_mr_cache_init(struct mlx5_ib_dev *dev)
 	int err;
 	int i;
 
+	mutex_init(&dev->slow_path_mutex);
 	cache->wq = create_singlethread_workqueue("mkey_cache");
 	if (!cache->wq) {
 		mlx5_ib_warn(dev, "failed to create work queue\n");
@@ -909,9 +910,12 @@ struct ib_mr *mlx5_ib_reg_user_mr(struct ib_pd *pd, u64 start, u64 length,
 		}
 	}
 
-	if (!mr)
+	if (!mr) {
+		mutex_lock(&dev->slow_path_mutex);
 		mr = reg_create(pd, virt_addr, length, umem, ncont, page_shift,
 				access_flags);
+		mutex_unlock(&dev->slow_path_mutex);
+	}
 
 	if (IS_ERR(mr)) {
 		err = PTR_ERR(mr);