locking/lockdep: Adjust lock usage bit character checks

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@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bvanassche@acm.org
Cc: frederic@kernel.org
Cc: ming.lei@redhat.com
Cc: will.deacon@arm.com
Link: https://lkml.kernel.org/r/20190506081939.74287-4-duyuyang@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 16 insertions, 5 deletions

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 109b56267c8f..a033df00fd1d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -500,15 +500,26 @@ 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;
 }