[alsa-devel] [PATCH 02/18] soundwire: fix race between driver probe and update_status callback

[Pierre-Louis Bossart]

The driver probe takes care of basic initialization and is invoked
when a Slave becomes attached, after a match between the Slave DevID
registers and ACPI/DT entries.

The update_status callback is invoked when a Slave state changes,
e.g. when it is assigned a non-zero Device Number and it reports with
an ATTACHED/ALERT state.

The state change detection is usually hardware-based and based on the
SoundWire frame rate (e.g. double-digit microseconds) while the probe
is a pure software operation, which may involve a kernel module
load. In corner cases, it's possible that the state changes before the
probe completes.

This patch suggests the use of wait_for_completion to avoid races on
startup, so that the update_status callback does not rely on invalid
pointers/data structures.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
---
 drivers/soundwire/bus.c      | 24 +++++++++++++++++++++---
 drivers/soundwire/bus.h      |  1 +
 drivers/soundwire/bus_type.c |  5 +++++
 drivers/soundwire/slave.c    |  2 ++
 4 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
index 4b22ee996a65..903aee258800 100644
--- a/drivers/soundwire/bus.c
+++ b/drivers/soundwire/bus.c
@@ -961,10 +961,28 @@ static int sdw_handle_slave_alerts(struct sdw_slave *slave)
 static int sdw_update_slave_status(struct sdw_slave *slave,
 				   enum sdw_slave_status status)
 {
-	if (slave->ops && slave->ops->update_status)
-		return slave->ops->update_status(slave, status);
+	unsigned long time;
 
-	return 0;
+	if (!slave->ops || !slave->ops->update_status)
+		return 0;
+
+	if (!slave->probed) {
+		/*
+		 * the slave status update is typically handled in an
+		 * interrupt thread, which can race with the driver
+		 * probe, e.g. when a module needs to be loaded.
+		 *
+		 * make sure the probe is complete before updating
+		 * status.
+		 */
+		time = wait_for_completion_timeout(&slave->probe_complete,
+				msecs_to_jiffies(DEFAULT_PROBE_TIMEOUT));
+		if (!time) {
+			dev_err(&slave->dev, "Probe not complete, timed out\n");
+			return -ETIMEDOUT;
+		}
+	}
+	return slave->ops->update_status(slave, status);
 }
 
 /**
diff --git a/drivers/soundwire/bus.h b/drivers/soundwire/bus.h
index cb482da914da..acb8d11a4c84 100644
--- a/drivers/soundwire/bus.h
+++ b/drivers/soundwire/bus.h
@@ -5,6 +5,7 @@
 #define __SDW_BUS_H
 
 #define DEFAULT_BANK_SWITCH_TIMEOUT 3000
+#define DEFAULT_PROBE_TIMEOUT       2000
 
 #if IS_ENABLED(CONFIG_ACPI)
 int sdw_acpi_find_slaves(struct sdw_bus *bus);
diff --git a/drivers/soundwire/bus_type.c b/drivers/soundwire/bus_type.c
index cf33f63773f0..3d52b72654c5 100644
--- a/drivers/soundwire/bus_type.c
+++ b/drivers/soundwire/bus_type.c
@@ -121,6 +121,11 @@ static int sdw_slave_drv_probe(struct device *dev)
 	slave->bus->clk_stop_timeout = max_t(u32, slave->bus->clk_stop_timeout,
 					     slave->prop.clk_stop_timeout);
 
+	slave->probed = true;
+	complete(&slave->probe_complete);
+
+	dev_dbg(dev, "probe complete\n");
+
 	return 0;
 }
 
diff --git a/drivers/soundwire/slave.c b/drivers/soundwire/slave.c
index c87267f12a3b..81b94cd3985e 100644
--- a/drivers/soundwire/slave.c
+++ b/drivers/soundwire/slave.c
@@ -52,6 +52,8 @@ static int sdw_slave_add(struct sdw_bus *bus,
 	slave->bus = bus;
 	slave->status = SDW_SLAVE_UNATTACHED;
 	slave->dev_num = 0;
+	init_completion(&slave->probe_complete);
+	slave->probed = false;
 
 	mutex_lock(&bus->bus_lock);
 	list_add_tail(&slave->node, &bus->slaves);
-- 
2.20.1