On 6/15/23 16:12, Richard Fitzgerald wrote:
> 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>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
> ---
> 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;
