linux-kernel - Re: [PATCH v2] random: do crng pre-init loading in worker rather than irq

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Message-ID: <YhzWE1NqYQFKeL5B@linutronix.de>
Date:   Mon, 28 Feb 2022 15:02:59 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Biggers <ebiggers@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v2] random: do crng pre-init loading in worker rather
 than irq

On 2022-02-24 16:29:37 [+0100], Jason A. Donenfeld wrote:
> Taking spinlocks from IRQ context is problematic for PREEMPT_RT. That
> is, in part, why we take trylocks instead. But apparently this still
> trips up various lock dependency analyzers. That seems like a bug in the
> analyzers that should be fixed, rather than having to change things
> here.

Could you please post a lockdep report so I can take a look?

> But maybe there's another reason to change things up: by deferring the
> crng pre-init loading to the worker, we can use the cryptographic hash
> function rather than xor, which is perhaps a meaningful difference when
> considering this data has only been through the relatively weak
> fast_mix() function.
> 
> The biggest downside of this approach is that the pre-init loading is
> now deferred until later, which means things that need random numbers
> after interrupts are enabled, but before workqueues are running -- or
> before this particular worker manages to run -- are going to get into
> trouble. Hopefully in the real world, this window is rather small,
> especially since this code won't run until 64 interrupts had occurred.
> 
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Eric Biggers <ebiggers@...nel.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

Other than that:

Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Sebastian