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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 29 Apr 2020 15:38:21 -0700
Message-ID: <CAHk-=wguiKq8yf11WJjgSL4ADKZ5sLe_Qbd7vHEqAkTvZJ+d+Q@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>
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On Wed, Apr 29, 2020 at 12:23 PM Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
>
> By the way, all other attempts to fix it look much more dangerous
> than my initially proposed patch, you know the one you hated, but
> it does work and does not look overly complicated either.

I don't think it works.

The whole "take lock, release it in the middle, then re-take it" is
fundamentally a broken model. We've never had it work well, and it
tends to have subtle issues. That's particularly true when you then
open-core the (only) acceptable sequence something like five times.

> What was the reason why that cannot be done this way?

If it had introduced a new locking model, and new locking helpers for
that model, with a comment in _one_ place, and nobody doing the ad-hoc
locking on their own, that might be more acceptable.

But that's not what that patch did. No way will I take something that
is so fragile and hacky, and repeats the hack N times.

If you do it properly, with a helper function instead of repeating
that fragile nasty thing, maybe it will look better to me.

That said, locks that get released in the middle aren't really locks.
But at least if the only way to take that lock had the "oh, this lock
is in that inconsistent state, I will return -EAGAIN", that would be
one thing. But when you have N different users and rely on all of them
getting that special semantic right, you're doing something wrong.

                Linus