RDMA/devices: Do not deadlock during client removal

lockdep reports:

   WARNING: possible circular locking dependency detected

   modprobe/302 is trying to acquire lock:
   0000000007c8919c ((wq_completion)ib_cm){+.+.}, at: flush_workqueue+0xdf/0x990

   but task is already holding lock:
   000000002d3d2ca9 (&device->client_data_rwsem){++++}, at: remove_client_context+0x79/0xd0 [ib_core]

   which lock already depends on the new lock.

   the existing dependency chain (in reverse order) is:

   -> #2 (&device->client_data_rwsem){++++}:
          down_read+0x3f/0x160
          ib_get_net_dev_by_params+0xd5/0x200 [ib_core]
          cma_ib_req_handler+0x5f6/0x2090 [rdma_cm]
          cm_process_work+0x29/0x110 [ib_cm]
          cm_req_handler+0x10f5/0x1c00 [ib_cm]
          cm_work_handler+0x54c/0x311d [ib_cm]
          process_one_work+0x4aa/0xa30
          worker_thread+0x62/0x5b0
          kthread+0x1ca/0x1f0
          ret_from_fork+0x24/0x30

   -> #1 ((work_completion)(&(&work->work)->work)){+.+.}:
          process_one_work+0x45f/0xa30
          worker_thread+0x62/0x5b0
          kthread+0x1ca/0x1f0
          ret_from_fork+0x24/0x30

   -> #0 ((wq_completion)ib_cm){+.+.}:
          lock_acquire+0xc8/0x1d0
          flush_workqueue+0x102/0x990
          cm_remove_one+0x30e/0x3c0 [ib_cm]
          remove_client_context+0x94/0xd0 [ib_core]
          disable_device+0x10a/0x1f0 [ib_core]
          __ib_unregister_device+0x5a/0xe0 [ib_core]
          ib_unregister_device+0x21/0x30 [ib_core]
          mlx5_ib_stage_ib_reg_cleanup+0x9/0x10 [mlx5_ib]
          __mlx5_ib_remove+0x3d/0x70 [mlx5_ib]
          mlx5_ib_remove+0x12e/0x140 [mlx5_ib]
          mlx5_remove_device+0x144/0x150 [mlx5_core]
          mlx5_unregister_interface+0x3f/0xf0 [mlx5_core]
          mlx5_ib_cleanup+0x10/0x3a [mlx5_ib]
          __x64_sys_delete_module+0x227/0x350
          do_syscall_64+0xc3/0x6a4
          entry_SYSCALL_64_after_hwframe+0x49/0xbe

Which is due to the read side of the client_data_rwsem being obtained
recursively through a work queue flush during cm client removal.

The lock is being held across the remove in remove_client_context() so
that the function is a fence, once it returns the client is removed. This
is required so that the two callers do not proceed with destruction until
the client completes removal.

Instead of using client_data_rwsem use the existing device unregistration
refcount and add a similar client unregistration (client->uses) refcount.

This will fence the two unregistration paths without holding any locks.

Cc: <stable@vger.kernel.org>
Fixes: 921eab1 ("RDMA/devices: Re-organize device.c locking")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20190731081841.32345-2-leon@kernel.org
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/device.c

@@ -99,6 +99,12 @@ static LIST_HEAD(client_list);
 static DEFINE_XARRAY_FLAGS(clients, XA_FLAGS_ALLOC);
 static DECLARE_RWSEM(clients_rwsem);
 
+static void ib_client_put(struct ib_client *client)
+{
+	if (refcount_dec_and_test(&client->uses))
+		complete(&client->uses_zero);
+}
+
 /*
  * If client_data is registered then the corresponding client must also still
  * be registered.
@@ -660,6 +666,14 @@ static int add_client_context(struct ib_device *device,
 		return 0;
 
 	down_write(&device->client_data_rwsem);
+	/*
+	 * So long as the client is registered hold both the client and device
+	 * unregistration locks.
+	 */
+	if (!refcount_inc_not_zero(&client->uses))
+		goto out_unlock;
+	refcount_inc(&device->refcount);
+
 	/*
 	 * Another caller to add_client_context got here first and has already
 	 * completely initialized context.
@@ -683,6 +697,9 @@ static int add_client_context(struct ib_device *device,
 	return 0;
 
 out:
+	ib_device_put(device);
+	ib_client_put(client);
+out_unlock:
 	up_write(&device->client_data_rwsem);
 	return ret;
 }
@@ -702,7 +719,7 @@ static void remove_client_context(struct ib_device *device,
 	client_data = xa_load(&device->client_data, client_id);
 	xa_clear_mark(&device->client_data, client_id, CLIENT_DATA_REGISTERED);
 	client = xa_load(&clients, client_id);
-	downgrade_write(&device->client_data_rwsem);
+	up_write(&device->client_data_rwsem);
 
 	/*
 	 * Notice we cannot be holding any exclusive locks when calling the
@@ -712,17 +729,13 @@ static void remove_client_context(struct ib_device *device,
 	 *
 	 * For this reason clients and drivers should not call the
 	 * unregistration functions will holdling any locks.
-	 *
-	 * It tempting to drop the client_data_rwsem too, but this is required
-	 * to ensure that unregister_client does not return until all clients
-	 * are completely unregistered, which is required to avoid module
-	 * unloading races.
 	 */
 	if (client->remove)
 		client->remove(device, client_data);
 
 	xa_erase(&device->client_data, client_id);
-	up_read(&device->client_data_rwsem);
+	ib_device_put(device);
+	ib_client_put(client);
 }
 
 static int alloc_port_data(struct ib_device *device)
@@ -1705,6 +1718,8 @@ int ib_register_client(struct ib_client *client)
 	unsigned long index;
 	int ret;
 
+	refcount_set(&client->uses, 1);
+	init_completion(&client->uses_zero);
 	ret = assign_client_id(client);
 	if (ret)
 		return ret;
@@ -1740,16 +1755,29 @@ void ib_unregister_client(struct ib_client *client)
 	unsigned long index;
 
 	down_write(&clients_rwsem);
+	ib_client_put(client);
 	xa_clear_mark(&clients, client->client_id, CLIENT_REGISTERED);
 	up_write(&clients_rwsem);
+
+	/* We do not want to have locks while calling client->remove() */
+	rcu_read_lock();
+	xa_for_each (&devices, index, device) {
+		if (!ib_device_try_get(device))
+			continue;
+		rcu_read_unlock();
+
+		remove_client_context(device, client->client_id);
+
+		ib_device_put(device);
+		rcu_read_lock();
+	}
+	rcu_read_unlock();
+
 	/*
-	 * Every device still known must be serialized to make sure we are
-	 * done with the client callbacks before we return.
+	 * remove_client_context() is not a fence, it can return even though a
+	 * removal is ongoing. Wait until all removals are completed.
 	 */
-	down_read(&devices_rwsem);
-	xa_for_each (&devices, index, device)
-		remove_client_context(device, client->client_id);
-	up_read(&devices_rwsem);
+	wait_for_completion(&client->uses_zero);
 
 	down_write(&clients_rwsem);
 	list_del(&client->list);

include/rdma/ib_verbs.h

@@ -2647,6 +2647,9 @@ struct ib_client {
 			const union ib_gid *gid,
 			const struct sockaddr *addr,
 			void *client_data);
+
+	refcount_t uses;
+	struct completion uses_zero;
 	struct list_head list;
 	u32 client_id;