Re: [BACKPORT 4.14.y v3 1/3] locking/lockdep: Add debug_locks check in __lock_downgrade()

[Waiman Long]

On 9/25/19 6:01 AM, Baolin Wang wrote:
> From: Waiman Long <longman@xxxxxxxxxx>
>
> [Upstream commit 513e1073d52e55b8024b4f238a48de7587c64ccf]
>
> 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@xxxxxxxxxxxxxxxxxxx>
> Reported-by: syzbot+53383ae265fb161ef488@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Link: https://lkml.kernel.org/r/1547093005-26085-1-git-send-email-longman@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxx>
> ---
>  kernel/locking/lockdep.c |    3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 565005a..5c370c6 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3650,6 +3650,9 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
>  	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,

Apparently, there are 2 such patches in the upstream kernel - commit
513e1073d52e55b8024b4f238a48de7587c64ccf and
71492580571467fb7177aade19c18ce7486267f5. These are probably caused by
the fact that there are 2 places in the code that can match the hunks.
Anyway, this looks like it is applying to the wrong function. It should
be applied to __lock_downgrade. Though it shouldn't harm if it is
applied to the wrong function.

Cheers,
Longman