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Date:   Fri, 10 May 2019 17:10:25 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     Christian Brauner <christian@brauner.io>,
        Daniel Colascione <dancol@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tim Murray <timmurray@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arve =?iso-8859-1?B?SGr4bm5lduVn?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>,
        kernel-team <kernel-team@android.com>,
        Andy Lutomirski <luto@amacapital.net>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Kees Cook <keescook@chromium.org>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
Message-ID: <20190510151024.GA21421@redhat.com>
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On 05/09, Sultan Alsawaf wrote:
>
> On Thu, May 09, 2019 at 05:56:46PM +0200, Oleg Nesterov wrote:
> > Impossible ;) I bet lockdep should report the deadlock as soon as find_victims()
> > calls find_lock_task_mm() when you already have a locked victim.
>
> I hope you're not a betting man ;)

I am starting to think I am ;)

If you have task1 != task2 this code

	task_lock(task1);
	task_lock(task2);

should trigger print_deadlock_bug(), task1->alloc_lock and task2->alloc_lock are
the "same" lock from lockdep pov, held_lock's will have the same hlock_class().

> CONFIG_PROVE_LOCKING=y

OK,

> And a printk added in vtsk_is_duplicate() to print when a duplicate is detected,

in this case find_lock_task_mm() won't be called, and this is what saves us from
the actual deadlock.


> and my phone's memory cut in half to make simple_lmk do something, this is what
> I observed:
> taimen:/ # dmesg | grep lockdep
> [    0.000000] \x09RCU lockdep checking is enabled.

this reports that CONFIG_PROVE_RCU is enabled ;)

> taimen:/ # dmesg | grep simple_lmk
> [   23.211091] simple_lmk: Killing android.carrier with adj 906 to free 37420 kiB
> [   23.211160] simple_lmk: Killing oadcastreceiver with adj 906 to free 36784 kiB

yes, looks like simple_lmk has at least 2 locked victims. And I have no idea why
you do not see anything else in dmesg. May be debug_locks_off() was already called.

But see above, "grep lockdep" won't work.  Perhaps you can do
"grep -e WARNING -e BUG -e locking".

Oleg.