linux-kernel - Re: [PATCH v4 1/2] random: remove batched entropy locking

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Message-ID: <YhL5Ur59uC+HQOY+@sol.localdomain>
Date:   Sun, 20 Feb 2022 18:30:42 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v4 1/2] random: remove batched entropy locking

On Wed, Feb 09, 2022 at 01:56:43PM +0100, Jason A. Donenfeld wrote:
> Rather than use spinlocks to protect batched entropy, we can instead
> disable interrupts locally, since we're dealing with per-cpu data, and
> manage resets with a basic generation counter. At the same time, we
> can't quite do this on PREEMPT_RT, where we still want spinlocks-as-
> mutexes semantics. So we use a local_lock_t, which provides the right
> behavior for each. Because this is a per-cpu lock, that generation
> counter is still doing the necessary CPU-to-CPU communication.
> 
> This should improve performance a bit. It will also fix the linked splat
> that Jonathan received with a PROVE_RAW_LOCK_NESTING=y.
> 
> Suggested-by: Andy Lutomirski <luto@...nel.org>
> Reported-by: Jonathan Neuschäfer <j.neuschaefer@....net>
> Tested-by: Jonathan Neuschäfer <j.neuschaefer@....net>
> Link: https://lore.kernel.org/lkml/YfMa0QgsjCVdRAvJ@latitude/
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Sultan Alsawaf <sultan@...neltoast.com>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  drivers/char/random.c | 55 ++++++++++++++++++++++---------------------
>  1 file changed, 28 insertions(+), 27 deletions(-)

Reviewed-by: Eric Biggers <ebiggers@...gle.com>

- Eric