linux-kernel - Re: [PATCH v6] random: defer fast pool mixing to worker

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Date:   Mon, 14 Feb 2022 11:17:20 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        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>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v6] random: defer fast pool mixing to worker

On 2/14/22, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> to
> | - Does anything anywhere call get_random_xx() before the worker has a
> |   chance to run?
>
> Once you queue a work item I don't think that the scheduler needs to put
> it on the CPU right away. It may have already have other tasks waiting
> including some with a RT priority.
> Also, the lock is irqsave() so they can be users in an interrupt
> handler. I remember the original reason why I made it irqsave is because
> something did kmalloc() and SLUB somehow asked for random bits.

Right. So there are two sides of the questions: 1) how bad is this
actual race, and are there any drivers that do regularly get bit by
this? 2) There's a largeish window between workqueue_init_early()
setting up the system highprio workqueue, and workqueue_init()
enabling queued workers to actually run. Interrupts also get enabled
in the interim. Does anything get bit by that window?

Jason