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

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Date:   Mon, 1 Aug 2022 14:46:35 +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 14:39:46 [+0200], Jason A. Donenfeld wrote:
> On RT, we can't call get_random_bytes() from inside of the raw locks
> that callers of vsprintf might take, because get_random_bytes() takes
> normal spinlocks. So on those RT systems, defer the siphash key
> generation to a worker.
> 
> Also, avoid using a static_branch, as this isn't the fast path.
> Using static_branch_likely() to signal that ptr_key has been filled is a
> bit much given that it is not a fast path.
> 
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Reported-by: Mike Galbraith <efault@....de>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> Sebastian - feel free to take this and tweak it as needed. Sending this
> mostly as something illustrative of what the "simpler" thing would be
> that I had in mind. -Jason

Can have the same behaviour regardless of CONFIG_PREEMPT_RT? Here
lockdep _may_ yell with !RT because it is broken for RT.
If we agree that we drop the first %p print here, can we do this on
both (regardless of CONFIG_PREEMPT_RT)?

Sebastian