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Date:   Thu, 16 May 2019 15:54:36 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sultan Alsawaf <sultan@...neltoast.com>
Cc:     Christian Brauner <christian@...uner.io>,
        Daniel Colascione <dancol@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Murray <timmurray@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>,
        kernel-team <kernel-team@...roid.com>,
        Andy Lutomirski <luto@...capital.net>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

On 05/15, Sultan Alsawaf wrote:
>
> On Wed, May 15, 2019 at 04:58:32PM +0200, Oleg Nesterov wrote:
> > Could you explain in detail what exactly did you do and what do you see in dmesg?
> >
> > Just in case, lockdep complains only once, print_circular_bug() does debug_locks_off()
> > so it it has already reported another false positive __lock_acquire() will simply
> > return after that.
> >
> > Oleg.
>
> This is what I did:
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 774ab79d3ec7..009e7d431a88 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3078,6 +3078,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>         int class_idx;
>         u64 chain_key;
>
> +       BUG_ON(!debug_locks || !prove_locking);
>         if (unlikely(!debug_locks))
>                 return 0;
>
> diff --git a/lib/debug_locks.c b/lib/debug_locks.c
> index 124fdf238b3d..4003a18420fb 100644
> --- a/lib/debug_locks.c
> +++ b/lib/debug_locks.c
> @@ -37,6 +37,7 @@ EXPORT_SYMBOL_GPL(debug_locks_silent);
>   */
>  int debug_locks_off(void)
>  {
> +       return 0;
>         if (debug_locks && __debug_locks_off()) {
>                 if (!debug_locks_silent) {
>                         console_verbose();

OK, this means that debug_locks_off() always returns 0, as if debug_locks was already
cleared.

Thus print_deadlock_bug() will do nothing, it does

	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
		return 0;

iow this means that even if lockdep finds a problem, the problem won't be reported.

> [    1.492128] BUG: key 0000000000000000 not in .data!
> [    1.492141] BUG: key 0000000000000000 not in .data!
> [    1.492152] BUG: key 0000000000000000 not in .data!
> [    1.492228] BUG: key 0000000000000000 not in .data!
> [    1.492238] BUG: key 0000000000000000 not in .data!
> [    1.492248] BUG: key 0000000000000000 not in .data!

I guess this is lockdep_init_map() which does printk("BUG:") itself, but due to your
change above it doesn't do WARN(1) and thus there is no call trace.

Oleg.

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