locking/lockdep: Move sanity check to inside lockdep_print_held_locks()

Calling lockdep_print_held_locks() on a running thread is considered unsafe.

Since all callers should follow that rule and the sanity check is not heavy,
this patch moves the sanity check to inside lockdep_print_held_locks().

As a side effect of this patch, the number of locks held by running threads
will be printed as well. This change will be preferable when we want to
know which threads might be relevant to a problem but are unable to print
any clues because that thread is running.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1523011279-8206-2-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -561,20 +561,24 @@ static void print_lock(struct held_lock *hlock)
 	printk(KERN_CONT ", at: %pS\n", (void *)hlock->acquire_ip);
 }
 
-static void lockdep_print_held_locks(struct task_struct *curr)
+static void lockdep_print_held_locks(struct task_struct *p)
 {
-	int i, depth = curr->lockdep_depth;
+	int i, depth = READ_ONCE(p->lockdep_depth);
 
-	if (!depth) {
-		printk("no locks held by %s/%d.\n", curr->comm, task_pid_nr(curr));
+	if (!depth)
+		printk("no locks held by %s/%d.\n", p->comm, task_pid_nr(p));
+	else
+		printk("%d lock%s held by %s/%d:\n", depth,
+		       depth > 1 ? "s" : "", p->comm, task_pid_nr(p));
+	/*
+	 * It's not reliable to print a task's held locks if it's not sleeping
+	 * and it's not the current task.
+	 */
+	if (p->state == TASK_RUNNING && p != current)
 		return;
-	}
-	printk("%d lock%s held by %s/%d:\n",
-		depth, depth > 1 ? "s" : "", curr->comm, task_pid_nr(curr));
-
 	for (i = 0; i < depth; i++) {
 		printk(" #%d: ", i);
-		print_lock(curr->held_locks + i);
+		print_lock(p->held_locks + i);
 	}
 }
 
@@ -4460,13 +4464,6 @@ void debug_show_all_locks(void)
 
 	rcu_read_lock();
 	for_each_process_thread(g, p) {
-		/*
-		 * It's not reliable to print a task's held locks
-		 * if it's not sleeping (or if it's not the current
-		 * task):
-		 */
-		if (p->state == TASK_RUNNING && p != current)
-			continue;
 		if (!p->lockdep_depth)
 			continue;
 		lockdep_print_held_locks(p);