[PATCH] spi: Fix deadlock when adding SPI controllers on SPI buses

[Mark Brown <broonie@xxxxxxxxxx>]

Currently we have a global spi_add_lock which we take when adding new
devices so that we can check that we're not trying to reuse a chip
select that's already controlled.  This means that if the SPI device is
itself a SPI controller and triggers the instantiation of further SPI
devices we trigger a deadlock as we try to register and instantiate
those devices while in the process of doing so for the parent controller
and hence already holding the global spi_add_lock.  Since we only care
about concurrency within a single SPI bus move the lock to be per
controller, avoiding the deadlock.

This can be easily triggered in the case of spi-mux.

Reported-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
 drivers/spi/spi.c       | 21 ++++++++++-----------
 include/linux/spi/spi.h |  3 +++
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 401f62f6f5b5..71d061132ada 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -518,12 +518,6 @@ static LIST_HEAD(spi_controller_list);
  */
 static DEFINE_MUTEX(board_lock);
 
-/*
- * Prevents addition of devices with same chip select and
- * addition of devices below an unregistering controller.
- */
-static DEFINE_MUTEX(spi_add_lock);
-
 /**
  * spi_alloc_device - Allocate a new SPI device
  * @ctlr: Controller to which device is connected
@@ -676,9 +670,13 @@ static int spi_add_device(struct spi_device *spi)
 	/* Set the bus ID string */
 	spi_dev_set_name(spi);
 
-	mutex_lock(&spi_add_lock);
+	/* We need to make sure there's no other device with this
+	 * chipselect **BEFORE** we call setup(), else we'll trash
+	 * its configuration.  Lock against concurrent add() calls.
+	 */
+	mutex_lock(&ctlr->add_lock);
 	status = __spi_add_device(spi);
-	mutex_unlock(&spi_add_lock);
+	mutex_unlock(&ctlr->add_lock);
 	return status;
 }
 
@@ -697,7 +695,7 @@ static int spi_add_device_locked(struct spi_device *spi)
 	/* Set the bus ID string */
 	spi_dev_set_name(spi);
 
-	WARN_ON(!mutex_is_locked(&spi_add_lock));
+	WARN_ON(!mutex_is_locked(&ctlr->add_lock));
 	return __spi_add_device(spi);
 }
 
@@ -2950,6 +2948,7 @@ int spi_register_controller(struct spi_controller *ctlr)
 	spin_lock_init(&ctlr->bus_lock_spinlock);
 	mutex_init(&ctlr->bus_lock_mutex);
 	mutex_init(&ctlr->io_mutex);
+	mutex_init(&ctlr->add_lock);
 	ctlr->bus_lock_flag = 0;
 	init_completion(&ctlr->xfer_completion);
 	if (!ctlr->max_dma_len)
@@ -3086,7 +3085,7 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 
 	/* Prevent addition of new devices, unregister existing ones */
 	if (IS_ENABLED(CONFIG_SPI_DYNAMIC))
-		mutex_lock(&spi_add_lock);
+		mutex_lock(&ctlr->add_lock);
 
 	device_for_each_child(&ctlr->dev, NULL, __unregister);
 
@@ -3117,7 +3116,7 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 	mutex_unlock(&board_lock);
 
 	if (IS_ENABLED(CONFIG_SPI_DYNAMIC))
-		mutex_unlock(&spi_add_lock);
+		mutex_unlock(&ctlr->add_lock);
 }
 EXPORT_SYMBOL_GPL(spi_unregister_controller);
 
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 29e21d49aafc..eb7ac8a1e03c 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -527,6 +527,9 @@ struct spi_controller {
 	/* I/O mutex */
 	struct mutex		io_mutex;
 
+	/* Used to avoid adding the same CS twice */
+	struct mutex		add_lock;
+
 	/* lock and mutex for SPI bus locking */
 	spinlock_t		bus_lock_spinlock;
 	struct mutex		bus_lock_mutex;
-- 
2.30.2