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

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Message-ID: <YufiYOOvnnioHK/1@linutronix.de>
Date:   Mon, 1 Aug 2022 16:25:36 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     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

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.
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.

> Jason

Sebastian