[tip:locking/core] locking/lockdep: Add debug_locks check in __lock_downgrade()

[tip-bot for Waiman Long <tipbot@zytor.com>]

Commit-ID:  513e1073d52e55b8024b4f238a48de7587c64ccf
Gitweb:     https://git.kernel.org/tip/513e1073d52e55b8024b4f238a48de7587c64ccf
Author:     Waiman Long <longman@redhat.com>
AuthorDate: Wed, 9 Jan 2019 23:03:25 -0500
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 4 Feb 2019 09:03:27 +0100

locking/lockdep: Add debug_locks check in __lock_downgrade()

Tetsuo Handa had reported he saw an incorrect "downgrading a read lock"
warning right after a previous lockdep warning. It is likely that the
previous warning turned off lock debugging causing the lockdep to have
inconsistency states leading to the lock downgrade warning.

Fix that by add a check for debug_locks at the beginning of
__lock_downgrade().

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Reported-by: syzbot+53383ae265fb161ef488@syzkaller.appspotmail.com
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: https://lkml.kernel.org/r/1547093005-26085-1-git-send-email-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/lockdep.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 608f74ed8bb9..7f7db23fc002 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3479,6 +3479,9 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
 	unsigned int depth;
 	int i;
 
+	if (unlikely(!debug_locks))
+		return 0;
+
 	depth = curr->lockdep_depth;
 	/*
 	 * This function is about (re)setting the class of a held lock,