bus: fsl-mc: fix dprc object reading race

When modifying the objects attached to a DPRC, we may end up reading
the list of objects from the firmware while another thread is changing
changing the list. Since we read the objects via:

- Read the number of DPRC objects
- Iterate over this number of objects retrieving their details

and objects can be added in the middle of the list, this causes the
last few objects to unexpectedly disappear. The side effect of this
is if network interfaces are added after boot, they come and go. This
can result in already configured interfaces unexpectedly disappearing.

This has been easy to provoke with the restool interface added, and a
script which adds network interfaces one after each other; the kernel
rescanning runs asynchronously to restool.

NXP's approach to fixing this was to introduce a sysfs "attribute" in
their vendor tree, /sys/bus/fsl-mc/rescan, which userspace poked at to
request the kernel to rescan the DPRC object tree each time the
"restool" command completed (whether or not the tool changed anything.)
This has the effect of making the kernel's rescan synchronous with a
scripted restool, but still fails if we have multiple restools running
concurrently.

This patch takes a different approach:
- Read the number of DPRC objects
- Iterate over this number of objects retrieving their details
- Re-read the number of DPRC objects
- If the number of DPRC objects has changed while reading, repeat.

This solves the issue where network interfaces unexpectedly disappear
while adding others via ls-addni, because they've fallen off the end
of the object list.

This does *not* solve the issue that if an object is deleted while
another is added while we are reading the objects - that requires
firmware modification, or a more elaborate solution on the Linux side
(e.g., CRCing the object details and reading all objects at least
twice to check the CRC is stable.)

However, without firmware modification, this is probably the best way
to ensure that we read all the objects.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 29 insertions, 3 deletions

diff --git a/drivers/bus/fsl-mc/dprc-driver.c b/drivers/bus/fsl-mc/dprc-driver.c
index e3e2ae41c22b..5647f60769ed 100644
--- a/drivers/bus/fsl-mc/dprc-driver.c
+++ b/drivers/bus/fsl-mc/dprc-driver.c
@@ -240,11 +240,11 @@ static void dprc_add_new_devices(struct fsl_mc_device *mc_bus_dev,
 int dprc_scan_objects(struct fsl_mc_device *mc_bus_dev,
 		      bool alloc_interrupts)
 {
-	int num_child_objects;
+	int num_child_objects, num_child_objects2;
 	int dprc_get_obj_failures;
 	int error;
-	unsigned int irq_count = mc_bus_dev->obj_desc.irq_count;
-	struct fsl_mc_obj_desc *child_obj_desc_array = NULL;
+	unsigned int irq_count;
+	struct fsl_mc_obj_desc *child_obj_desc_array;
 	struct fsl_mc_bus *mc_bus = to_fsl_mc_bus(mc_bus_dev);
 
 	error = dprc_get_obj_count(mc_bus_dev->mc_io,
@@ -257,6 +257,9 @@ int dprc_scan_objects(struct fsl_mc_device *mc_bus_dev,
 		return error;
 	}
 
+retry:
+	irq_count = mc_bus_dev->obj_desc.irq_count;
+	child_obj_desc_array = NULL;
 	if (num_child_objects != 0) {
 		int i;
 
@@ -315,6 +318,29 @@ int dprc_scan_objects(struct fsl_mc_device *mc_bus_dev,
 		}
 	}
 
+	error = dprc_get_obj_count(mc_bus_dev->mc_io,
+				   0,
+				   mc_bus_dev->mc_handle,
+				   &num_child_objects2);
+	if (error < 0) {
+		if (child_obj_desc_array)
+			devm_kfree(&mc_bus_dev->dev, child_obj_desc_array);
+		dev_err(&mc_bus_dev->dev, "dprc_get_obj_count() failed: %d\n",
+			error);
+		return error;
+	}
+
+	if (num_child_objects != num_child_objects2) {
+		/*
+		 * Something changed while reading the number of objects.
+		 * Retry reading the child object list.
+		 */
+		if (child_obj_desc_array)
+			devm_kfree(&mc_bus_dev->dev, child_obj_desc_array);
+		num_child_objects = num_child_objects2;
+		goto retry;
+	}
+
 	/*
 	 * Allocate IRQ's before binding the scanned devices with their
 	 * respective drivers.