linux-kernel - Re: [PATCH v4] lib/vsprintf: defer filling siphash key on RT

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Message-ID: <CAHmME9qPVszZqgDBhUUMMhnbg8y1-1rVC+rikPuwt+B-=7P4Lw@mail.gmail.com>
Date:   Mon, 1 Aug 2022 16:30:03 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mike Galbraith <efault@....de>, Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] lib/vsprintf: defer filling siphash key on RT

Hi Sebastian,

On Mon, Aug 1, 2022 at 4:25 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2022-08-01 15:44:12 [+0200], Jason A. Donenfeld wrote:
> > Hey again,
> Hi Jason,
>
> > By the way, another option that would be fine with me would be to make
> > random.c use all raw spinlocks. From a non-RT perspective, that wouldn't
> > change the codegen at all, so it doesn't make a huge difference to me.
> > From an RT perspective, it would presumably fix a lot of these issues,
> > and enable randomness to be available in any context, which is maybe
> > what we want anyway. From an RT-safety point of view, I suspect doing
> > this might actually be okay, because the locks are only ever protecting
> > operations that are fixed duration CPU-bound, like generating a chacha
> > block or something, not waiting for some I/O.
> >
> > Thoughts on that?
>
> That random-core change regarding random numbers broke lockdep, kasan (I
> think) and now printk's %p. Each one of them appears to be exceptional
> since we don't have _that_ many users asking for random numbers in
> atomic context.

Actually, the printk %p case was caused by something different than
the other. This used to be initialized with a clunky notifier callback
mechanism, which I got rid of, replacing it with this direct thing.
It's this direct thing that's now causing problems on RT.

> Making the locks raw would indeed solve all the issues at once. Last
> time I was looking into this, would include three locks and I tried to
> trigger the worst-case via "re-seed" and this was visible back then.
> After the rework you did back thinks looked good.

I actually just sent a patch for that to you a second ago:
https://lore.kernel.org/lkml/20220801142530.133007-1-Jason@zx2c4.com/

It's only two locks, and usage seems pretty constrained in a good way.
So okay, if you're on board, let's just do that, and then printk can
stay the same.

Jason