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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 20 Dec 2021 15:38:58 +0100
Message-ID: <CAHmME9pzdXyD0oRYyCoVUSqqsA9h03-oR7kcNhJuPEcEMTJYgw@mail.gmail.com>
Subject: Re: [PATCH 5/5] random: Defer processing of randomness on PREEMPT_RT.
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Herbert Xu <herbert@gondor.apana.org.au>
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Hey Sebastian,

Sorry for the delay getting back to you.

I think I understand the motivation for this patchset, and maybe it'll
turn out to be the only way of accomplishing what RT needs. But I
really don't like complicating the irq ingestion flow like that,
splitting the captured state into two pieces, and having multiple
entry points. It makes the whole thing more difficult to analyze and
maintain. Again, maybe we'll *have* to do this ultimately, but I want
to make sure we at least explore the alternatives fully.

One thing you brought up is that the place where a spinlock becomes
problematic for RT is if userspace goes completely nuts with
RNDRESEEDCRNG. If that's really the only place where contention might
be harmful, can't we employ other techniques there instead? For
example, just ratelimiting the frequency at which RNDRESEEDCRNG can be
called _before_ taking that lock, using the usual ratelimit.h
structure? Or, alternatively, what about a lock that very heavily
prioritizes acquisitions from the irq path rather than from the
userspace path? I think Herbert might have had a suggestion there
related to the net stack's sock lock?

Jason