lockdep: Reduce the size of lock_list::distance

lock_list::distance is always not greater than MAX_LOCK_DEPTH (which
is 48 right now), so a u16 will fit. This patch reduces the size of
lock_list::distance to save space, so that we can introduce other fields
to help detect recursive read lock deadlocks without increasing the size
of lock_list structure.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200807074238.1632519-6-boqun.feng@gmail.com

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 7cae5ea..2275010 100644 (file)
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -54,7 +54,7 @@ struct lock_list {
        struct lock_class               *class;
        struct lock_class               *links_to;
        const struct lock_trace         *trace;
-       int                             distance;
+       u16                             distance;
 
        /*
         * The parent field is used to implement breadth-first search, and the

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 150686a..668a983 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1320,7 +1320,7 @@ static struct lock_list *alloc_list_entry(void)
  */
 static int add_lock_to_list(struct lock_class *this,
                            struct lock_class *links_to, struct list_head *head,
-                           unsigned long ip, int distance,
+                           unsigned long ip, u16 distance,
                            const struct lock_trace *trace)
 {
        struct lock_list *entry;
@@ -2489,7 +2489,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next)
  */
 static int
 check_prev_add(struct task_struct *curr, struct held_lock *prev,
-              struct held_lock *next, int distance,
+              struct held_lock *next, u16 distance,
               struct lock_trace **const trace)
 {
        struct lock_list *entry;
@@ -2622,7 +2622,7 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
                goto out_bug;
 
        for (;;) {
-               int distance = curr->lockdep_depth - depth + 1;
+               u16 distance = curr->lockdep_depth - depth + 1;
                hlock = curr->held_locks + depth - 1;
 
                /*