[PATCH v2 03/23] locking/lockdep: Adjust lock usage bit character checks

[Yuyang Du]

The lock usage bit characters are defined and determined with tricks.
Add some explanation to make it a bit clearer, then adjust the logic to
check the usage, which optimizes the code a bit.

No functional change.

Signed-off-by: Yuyang Du <duyuyang@xxxxxxxxx>
---
 kernel/locking/lockdep.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a019330..720d195 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -499,15 +499,26 @@ static inline unsigned long lock_flag(enum lock_usage_bit bit)
 
 static char get_usage_char(struct lock_class *class, enum lock_usage_bit bit)
 {
+	/*
+	 * The usage character defaults to '.' (i.e., irqs disabled and not in
+	 * irq context), which is the safest usage category.
+	 */
 	char c = '.';
 
-	if (class->usage_mask & lock_flag(bit + LOCK_USAGE_DIR_MASK))
+	/*
+	 * The order of the following usage checks matters, which will
+	 * result in the outcome character as follows:
+	 *
+	 * - '+': irq is enabled and not in irq context
+	 * - '-': in irq context and irq is disabled
+	 * - '?': in irq context and irq is enabled
+	 */
+	if (class->usage_mask & lock_flag(bit + LOCK_USAGE_DIR_MASK)) {
 		c = '+';
-	if (class->usage_mask & lock_flag(bit)) {
-		c = '-';
-		if (class->usage_mask & lock_flag(bit + LOCK_USAGE_DIR_MASK))
+		if (class->usage_mask & lock_flag(bit))
 			c = '?';
-	}
+	} else if (class->usage_mask & lock_flag(bit))
+		c = '-';
 
 	return c;
 }
-- 
1.8.3.1