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

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Message-ID: <Yhzl7hgV12an3dhR@zx2c4.com>
Date:   Mon, 28 Feb 2022 16:10:38 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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

Hi Sebastian,

On Mon, Feb 28, 2022 at 03:29:32PM +0100, Sebastian Andrzej Siewior wrote:
> > > Could you please post a lockdep report so I can take a look?
> > 
> > I thought the problem with lockdep was stated by you somewhere in this thread?
> > https://lore.kernel.org/lkml/YfOqsOiNfURyvFRX@linutronix.de/
> > "But even then we need to find a way to move the crng init part
> > (crng_fast_load()) out of the hard-IRQ."
> > And Jonathan posted two related (?) splats he ran into.
> > 
> > I may have gotten that all wrong, in which case, I'll just excise that
> > part from the commit message. I'm pretty sure you want this patch
> > either way, right?
> 
> Oh, that report. So yes, I want that patch ;)
> 
> In this case the lockdep is right. The thing that it affects only
> PREEMPT_RT.
> That trylock is not the thing that lockdep complains about but the
> spin_lock_irqsave() within invalidate_batched_entropy().
> 
> Taking a spinlock_t from IRQ context is problematic for PREEMPT_RT,
> correct. A spin_try_lock() is also problematic since another spin_lock()
> invocation would PI-boost the wrong task (the spin_try_lock() is invoked
> from an IRQ-context so the task on CPU (random task or idle) is not the
> actual owner). I'm pointing this out because there was also _another_
> problem with try_lock from hard-IRQ context which was fixed in the
> meantime.
> 
> Would it work for you to update the commit message? Basically I'm fine
> with the firs sentence but the remaining part is misleading.

Ahh, I understand, okay. Yes, I'll change that first paragraph to
incorporate your wording, as:

"""
Taking spinlocks from IRQ context is generally problematic for
PREEMPT_RT. That is, in part, why we take trylocks instead. However, a
spin_try_lock() is also problematic since another spin_lock() invocation
can potentially PI-boost the wrong task, as the spin_try_lock() is
invoked from an IRQ-context, so the task on CPU (random task or idle) is
not the actual owner.
"""

Jason