[PATCH 1/2] soundwire: bus: Prevent lockdep asserts when stream has multiple buses

[Richard Fitzgerald]

Give the bus_lock and msg_lock of each bus a different unique key
so that it is possible to acquire the locks of multiple buses
without lockdep asserting a possible deadlock.

Using mutex_init() to initialize a mutex gives all those mutexes
the same lock class. Lockdep checking treats it as an error to
attempt to take a mutex while already holding a mutex of the same
class. This causes a lockdep assert when sdw_acquire_bus_lock()
attempts to lock multiple buses, and when do_bank_switch() takes
multiple msg_lock.

[  138.697350] WARNING: possible recursive locking detected
[  138.697366] 6.3.0-test #1 Tainted: G            E
[  138.697380] --------------------------------------------
[  138.697394] play/903 is trying to acquire lock:
[  138.697409] ffff99b8c41aa8c8 (&bus->bus_lock){+.+.}-{3:3}, at:
sdw_prepare_stream+0x52/0x2e0
[  138.697443]
               but task is already holding lock:
[  138.697468] ffff99b8c41af8c8 (&bus->bus_lock){+.+.}-{3:3}, at:
sdw_prepare_stream+0x52/0x2e0
[  138.697493]
               other info that might help us debug this:
[  138.697521]  Possible unsafe locking scenario:

[  138.697540]        CPU0
[  138.697550]        ----
[  138.697559]   lock(&bus->bus_lock);
[  138.697570]   lock(&bus->bus_lock);
[  138.697581]
                *** DEADLOCK ***

Giving each mutex a unique key allows multiple to be held
without triggering a lockdep assert. But note that it does not
allow them to be taken in one order then a different order.
If two mutexes are taken in the order A, B then they must
always be taken in that order otherwise they could deadlock.

Signed-off-by: Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx>
---
 drivers/soundwire/bus.c       | 15 +++++++++++++--
 include/linux/soundwire/sdw.h |  3 +++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
index b44f8d0affa6..dba920ec88f6 100644
--- a/drivers/soundwire/bus.c
+++ b/drivers/soundwire/bus.c
@@ -69,8 +69,17 @@ int sdw_bus_master_add(struct sdw_bus *bus, struct device *parent,
 		return -EINVAL;
 	}
 
-	mutex_init(&bus->msg_lock);
-	mutex_init(&bus->bus_lock);
+	/*
+	 * Give each bus_lock and msg_lock a unique key so that lockdep won't
+	 * trigger a deadlock warning when the locks of several buses are
+	 * grabbed during configuration of a multi-bus stream.
+	 */
+	lockdep_register_key(&bus->msg_lock_key);
+	__mutex_init(&bus->msg_lock, "msg_lock", &bus->msg_lock_key);
+
+	lockdep_register_key(&bus->bus_lock_key);
+	__mutex_init(&bus->bus_lock, "bus_lock", &bus->bus_lock_key);
+
 	INIT_LIST_HEAD(&bus->slaves);
 	INIT_LIST_HEAD(&bus->m_rt_list);
 
@@ -181,6 +190,8 @@ void sdw_bus_master_delete(struct sdw_bus *bus)
 	sdw_master_device_del(bus);
 
 	sdw_bus_debugfs_exit(bus);
+	lockdep_unregister_key(&bus->bus_lock_key);
+	lockdep_unregister_key(&bus->msg_lock_key);
 	ida_free(&sdw_bus_ida, bus->id);
 }
 EXPORT_SYMBOL(sdw_bus_master_delete);
diff --git a/include/linux/soundwire/sdw.h b/include/linux/soundwire/sdw.h
index c076a3f879b3..f523ceabd059 100644
--- a/include/linux/soundwire/sdw.h
+++ b/include/linux/soundwire/sdw.h
@@ -5,6 +5,7 @@
 #define __SOUNDWIRE_H
 
 #include <linux/bug.h>
+#include <linux/lockdep_types.h>
 #include <linux/mod_devicetable.h>
 #include <linux/bitfield.h>
 
@@ -907,7 +908,9 @@ struct sdw_bus {
 	struct list_head slaves;
 	DECLARE_BITMAP(assigned, SDW_MAX_DEVICES);
 	struct mutex bus_lock;
+	struct lock_class_key bus_lock_key;
 	struct mutex msg_lock;
+	struct lock_class_key msg_lock_key;
 	int (*compute_params)(struct sdw_bus *bus);
 	const struct sdw_master_ops *ops;
 	const struct sdw_master_port_ops *port_ops;
-- 
2.30.2