firewire: enforce access order between generation and node ID, fix "giving up on config rom"

fw_device.node_id and fw_device.generation are accessed without mutexes.
We have to ensure that all readers will get to see node_id updates
before generation updates.

Fixes an inability to recognize devices after "giving up on config rom",
https://bugzilla.redhat.com/show_bug.cgi?id=429950

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

Reviewed by Nick Piggin <nickpiggin@yahoo.com.au>.

Verified to fix 'giving up on config rom' issues on multiple system and
drive combinations that were previously affected.

Signed-off-by: Jarod Wilson <jwilson@redhat.com>
Signed-off-by: Kristian Høgsberg <krh@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/firewire/fw-device.h b/drivers/firewire/fw-device.h
index 894d4a92a18e..0854fe2bc110 100644
--- a/drivers/firewire/fw-device.h
+++ b/drivers/firewire/fw-device.h
@@ -35,6 +35,18 @@ struct fw_attribute_group {
         struct attribute *attrs[11];
 };
 
+/*
+ * Note, fw_device.generation always has to be read before fw_device.node_id.
+ * Use SMP memory barriers to ensure this.  Otherwise requests will be sent
+ * to an outdated node_id if the generation was updated in the meantime due
+ * to a bus reset.
+ *
+ * Likewise, fw-core will take care to update .node_id before .generation so
+ * that whenever fw_device.generation is current WRT the actual bus generation,
+ * fw_device.node_id is guaranteed to be current too.
+ *
+ * The same applies to fw_device.card->node_id vs. fw_device.generation.
+ */
 struct fw_device {
         atomic_t state;
         struct fw_node *node;