[PATCH 2/2] soundwire: stream: Make master_list ordered to prevent deadlocks

[Richard Fitzgerald]

Always add buses to the stream->master_list in a fixed order.
The unique bus->id is used to order the adding of buses to the
list.

This prevents lockdep asserts and possible deadlocks on streams
that have multiple buses.

sdw_acquire_bus_lock() takes bus_lock in the order that buses
are listed in stream->master_list. do_bank_switch() takes all
the msg_lock in the same order.

To prevent a lockdep assert, and a possible real deadlock, the
relative order of taking these mutexes must always be the same.

For example, if a stream takes the mutexes in the order
(bus0, bus1) lockdep will assert if another stream takes them
in the order (bus1, bus0).

More complex relative ordering will also assert, for example
if two streams take (bus0, bus1) and (bus1, bus2), then a third
stream takes (bus2, bus0).

Previously sdw_stream_add_master() simply added the given bus
to the end of the list, requiring the caller to guarantee that
buses are added in a fixed order. This isn't reasonable or
necessary - it's an internal implementation detail that should
not be exposed by the API. It doesn't really make sense when
there could be multiple independent calling drivers, to say
"you must add your buses in the same order as a different driver,
that you don't know about, added them".

Signed-off-by: Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx>
---
 drivers/soundwire/stream.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/soundwire/stream.c b/drivers/soundwire/stream.c
index 93baca08a0de..d77a8a0d42c8 100644
--- a/drivers/soundwire/stream.c
+++ b/drivers/soundwire/stream.c
@@ -1150,7 +1150,8 @@ static struct sdw_master_runtime
 *sdw_master_rt_alloc(struct sdw_bus *bus,
 		     struct sdw_stream_runtime *stream)
 {
-	struct sdw_master_runtime *m_rt;
+	struct sdw_master_runtime *m_rt, *walk_m_rt;
+	struct list_head *insert_after;
 
 	m_rt = kzalloc(sizeof(*m_rt), GFP_KERNEL);
 	if (!m_rt)
@@ -1159,7 +1160,20 @@ static struct sdw_master_runtime
 	/* Initialization of Master runtime handle */
 	INIT_LIST_HEAD(&m_rt->port_list);
 	INIT_LIST_HEAD(&m_rt->slave_rt_list);
-	list_add_tail(&m_rt->stream_node, &stream->master_list);
+
+	/*
+	 * Add in order of bus id so that when taking the bus_lock
+	 * of multiple buses they will always be taken in the same
+	 * order to prevent a mutex deadlock.
+	 */
+	insert_after = &stream->master_list;
+	list_for_each_entry_reverse(walk_m_rt, &stream->master_list, stream_node) {
+		if (walk_m_rt->bus->id < bus->id) {
+			insert_after = &walk_m_rt->stream_node;
+			break;
+		}
+	}
+	list_add(&m_rt->stream_node, insert_after);
 
 	list_add_tail(&m_rt->bus_node, &bus->m_rt_list);
 
-- 
2.30.2