Re: possible circular locking dependency detected [was: linux-next: Tree for Aug 22]

Byungchul Park
2017-08-23T05:44:16Z

On Wed, Aug 23, 2017 at 12:38:13PM +0800, Boqun Feng wrote:
> From: Boqun Feng <boqun.feng@gmail.com>
> Date: Wed, 23 Aug 2017 12:12:16 +0800
> Subject: [PATCH] lockdep: Print proper scenario if cross deadlock detected at
> acquisition time
>
> For a potential deadlock about CROSSRELEASE as follow:
>
> P1 P2
> =========== =============
> lock(A)
> lock(X)
> lock(A)
> commit(X)
>
> A: normal lock, X: cross lock
>
> , we could detect it at two places:
>
> 1. commit time:
>
> We have run P1 first, and have dependency A --> X in graph, and
> then we run P2, and find the deadlock.
>
> 2. acquisition time:
>
> We have run P2 first, and have dependency A --> X, in

X -> A

> graph(because another P3 may run previously and is acquiring for

".. another P3 may have run previously and was holding .."
^
Additionally, not only P3 but also P2 like:

lock(A)
lock(X)
lock(X) // I mean it's at _P2_
lock(A)
commit(X)

> lock X), and then we run P1 and find the deadlock.
>
> In current print_circular_lock_scenario(), for 1) we could print the
> right scenario and note that's a deadlock related to CROSSRELEASE,
> however for 2) we print the scenario as a normal lockdep deadlock.
>
> It's better to print a proper scenario related to CROSSRELEASE to help
> users find their bugs more easily, so improve this.
>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> kernel/locking/lockdep.c | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 642fb5362507..a3709e15f609 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1156,6 +1156,23 @@ print_circular_lock_scenario(struct held_lock *src,
> __print_lock_name(target);
> printk(KERN_CONT ");\n");
> printk("\n *** DEADLOCK ***\n\n");
> + } else if (cross_lock(src->instance)) {
> + printk(" Possible unsafe locking scenario by crosslock:\n\n");
> + printk(" CPU0 CPU1\n");
> + printk(" ---- ----\n");
> + printk(" lock(");
> + __print_lock_name(target);
> + printk(KERN_CONT ");\n");
> + printk(" lock(");
> + __print_lock_name(source);
> + printk(KERN_CONT ");\n");
> + printk(" lock(");
> + __print_lock_name(parent == source ? target : parent);
> + printk(KERN_CONT ");\n");
> + printk(" unlock(");
> + __print_lock_name(source);
> + printk(KERN_CONT ");\n");
> + printk("\n *** DEADLOCK ***\n\n");
> } else {
> printk(" Possible unsafe locking scenario:\n\n");
> printk(" CPU0 CPU1\n");

I need time to be sure if it's correct.