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

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Message-ID: <Ygo6EX1YVgTrtjVT@linutronix.de>
Date:   Mon, 14 Feb 2022 12:16:33 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
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 2022-02-14 11:17:20 [+0100], Jason A. Donenfeld wrote:
> 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?

This is only important during boot-up, right? Otherwise it just extracts
entropy from the pool.
I posted numbers earlier on where the work go scheduled and the three or
four interrupts came in before the work-item was scheduled. I could send
you the diff if you want to up it on some machines.
 
> Jason

Sebastian