linux-kernel - [PATCH v2 3/3] locking/lockdep: Print possible warning after counting deps

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Date:   Fri, 18 Jun 2021 22:55:34 +0800
From:   Xiongwei Song <sxwjean@...com>
To:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, Xiongwei Song <sxwjean@...il.com>
Subject: [PATCH v2 3/3] locking/lockdep: Print possible warning after counting deps

From: Xiongwei Song <sxwjean@...il.com>

The graph walk might hit error when counting dependencies. Once the
return value is negative, print a warning to reminder users.

However, lockdep_unlock() would be called twice if we call print_bfs_bug()
directly in __lockdep_count_*_deps(), so as the suggestion from Boqun:
"
Here print_bfs_bug() will eventually call debug_locks_off_graph_unlock()
to release the graph lock, and the caller (lockdep_count_fowards_deps())
will also call graph_unlock() afterwards, and that means we unlock
*twice* if a BFS error happens... although in that case, lockdep should
stop working so messing up with the graph lock may not hurt anything,
but still, I don't think we want to do that.

So probably you can open-code __lockdep_count_forward_deps() into
lockdep_count_forwards_deps(), and call print_bfs_bug() or
graph_unlock() accordingly. The body of __lockdep_count_forward_deps()
is really small, so I think it's OK to open-code it into its caller.
"
we put the code in __lockdep_count_*_deps() into lockdep_count_*_deps().

Suggested-by: Waiman Long <longman@...hat.com>
Suggested-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Xiongwei Song <sxwjean@...il.com>
---
 kernel/locking/lockdep.c | 45 +++++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 24 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index cb94097014d8..c29453b1df50 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2024,55 +2024,52 @@ static bool noop_count(struct lock_list *entry, void *data)
 	return false;
 }
 
-static unsigned long __lockdep_count_forward_deps(struct lock_list *this)
-{
-	unsigned long  count = 0;
-	struct lock_list *target_entry;
-
-	__bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
-
-	return count;
-}
 unsigned long lockdep_count_forward_deps(struct lock_class *class)
 {
-	unsigned long ret, flags;
+	unsigned long count = 0, flags;
 	struct lock_list this;
+	struct lock_list *target_entry;
+	enum bfs_result result;
 
 	__bfs_init_root(&this, class);
 
 	raw_local_irq_save(flags);
 	lockdep_lock();
-	ret = __lockdep_count_forward_deps(&this);
-	lockdep_unlock();
-	raw_local_irq_restore(flags);
 
-	return ret;
-}
+	result = __bfs_forwards(&this, (void *)&count, noop_count, NULL, &target_entry);
 
-static unsigned long __lockdep_count_backward_deps(struct lock_list *this)
-{
-	unsigned long  count = 0;
-	struct lock_list *target_entry;
+	if (bfs_error(result))
+		print_bfs_bug(result);
+	else
+		lockdep_unlock();
 
-	__bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
+	raw_local_irq_restore(flags);
 
 	return count;
 }
 
 unsigned long lockdep_count_backward_deps(struct lock_class *class)
 {
-	unsigned long ret, flags;
+	unsigned long  count = 0, flags;
 	struct lock_list this;
+	struct lock_list *target_entry;
+	enum bfs_result result;
 
 	__bfs_init_root(&this, class);
 
 	raw_local_irq_save(flags);
 	lockdep_lock();
-	ret = __lockdep_count_backward_deps(&this);
-	lockdep_unlock();
+
+	result = __bfs_backwards(&this, (void *)&count, noop_count, NULL, &target_entry);
+
+	if (bfs_error(result))
+		print_bfs_bug(result);
+	else
+		lockdep_unlock();
+
 	raw_local_irq_restore(flags);
 
-	return ret;
+	return count;
 }
 
 /*
-- 
2.30.2