firewire: sbp2: protect a reference counter properly

The assertion in the comment in sbp2_allow_block() is no longer true.
Or maybe it never was true.  At least now, the sole caller of
sbp2_allow_block(), sbp2_login, can run concurrently to one of
sbp2_unblock()'s callers, sbp2_remove.

sbp2_login is performed by sbp2_logical_unit.work.
sbp2_remove is performed by fw_device.work.
sbp2_remove cancels sbp2_logical_unit.work, but only after it called
sbp2_unblock.

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

1 files changed, 6 insertions, 8 deletions

diff --git a/drivers/firewire/sbp2.c b/drivers/firewire/sbp2.c
index 7aef911fdc71..c7fc78c23978 100644
--- a/drivers/firewire/sbp2.c
+++ b/drivers/firewire/sbp2.c
@@ -689,14 +689,12 @@ static void sbp2_agent_reset_no_wait(struct sbp2_logical_unit *lu)
 
 static inline void sbp2_allow_block(struct sbp2_logical_unit *lu)
 {
-        /*
-         * We may access dont_block without taking card->lock here:
-         * All callers of sbp2_allow_block() and all callers of sbp2_unblock()
-         * are currently serialized against each other.
-         * And a wrong result in sbp2_conditionally_block()'s access of
-         * dont_block is rather harmless, it simply misses its first chance.
-         */
-        --lu->tgt->dont_block;
+        struct sbp2_target *tgt = lu->tgt;
+        struct fw_card *card = target_parent_device(tgt)->card;
+
+        spin_lock_irq(&card->lock);
+        --tgt->dont_block;
+        spin_unlock_irq(&card->lock);
 }
 
 /*