Re: Deadlock in spi_add_device() -- device core problem

[Mark Brown <broonie@kernel.org>]

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On Thu, Oct 07, 2021 at 06:52:14PM +0200, Uwe Kleine-König wrote:
> On Thu, Oct 07, 2021 at 06:19:46PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, Oct 07, 2021 at 06:05:24PM +0200, Uwe Kleine-König wrote:

> > Drivers for a bus bind to the bus, they should not be creating new
> > devices for that same bus, as that does not seem correct.

> That's not the culprit here. We have a spi-device (spi-mux) that is a
> bus device on the SoC's bus and a bus master for it's own bus. And
> spi_add_device for the spi-mux device triggers the probe function for
> the spi-mux driver which creates a new bus controller which triggers
> spi_add_device() for the devices on the inner bus.

I think we need to be arranging for spi_add_lock to be per bus
rather than global - putting it into the controller ought to do
the trick.  We are only concerned about collisions on the same
bus, we don't care about other buses, so we don't need a global
lock and if the lock is per controller then we should't have any
issue adding a new bus in the middle of adding a device on
another bus.  Does the below totally untested patch work for you?

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index aea037c65985..412a10586233 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -478,12 +478,6 @@ 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
@@ -636,9 +630,9 @@ int spi_add_device(struct spi_device *spi)
 	 * chipselect **BEFORE** we call setup(), else we'll trash
 	 * its configuration.  Lock against concurrent add() calls.
 	 */
-	mutex_lock(&spi_add_lock);
+	mutex_lock(&ctlr->add_lock);
 	status = __spi_add_device(spi);
-	mutex_unlock(&spi_add_lock);
+	mutex_unlock(&ctlr->add_lock);
 	return status;
 }
 EXPORT_SYMBOL_GPL(spi_add_device);
@@ -658,7 +652,7 @@ 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);
 }
 
@@ -2830,6 +2824,7 @@ 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)
@@ -2966,7 +2961,7 @@ 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);
 
@@ -2997,7 +2992,7 @@ 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 8371bca13729..6b0b686f6f90 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -531,6 +531,9 @@ 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;

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