linux-kernel - Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

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Message-ID: <20181203083942.GF31738@dhcp22.suse.cz>
Date:   Mon, 3 Dec 2018 09:39:42 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Chanho Min <chanho.min@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux
 4.20-rc4)

On Mon 03-12-18 08:47:00, Ingo Molnar wrote:
[...]
> I reviewed the ->cred_guard_mutex code, and the mutex is held across all 
> of exec() - and we always did this.

Yes, this is something that has been pointed out during the review. Oleg
has argued that making this path freezable is really hard and that we
should be changing de_thread to sleep withtou cred_guard_mutex long term
anyway (http://lkml.kernel.org/r/20181114143705.GB13885@redhat.com).

Failing suspend seems like a real problem while the lockdep one doesn't
really reflect any real deadlock, right? So while the patch is not
perfect it shouldn't make the situation much worse. Lockdep splat is
certainly annoying but is it any worse than a suspend failing?

Now, I wouldn't mind to revert this because the code is really old and
we haven't seen many bug reports about failing suspend yet. But what is
the actual plan to make this work properly? Use
freezable_schedule_unsafe instead? Freezer code has some
fundamental design issues which are quite hard to get over.
-- 
Michal Hocko
SUSE Labs