linux-kernel - Re: [PATCH v2] random: set fast pool count to zero in cpuhp teardown

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Message-ID: <CAHmME9rp+O3+yjX_q-BS8y06PigbkEgi4vn=nzLACnBAWZt-vA@mail.gmail.com>
Date:   Mon, 14 Feb 2022 15:52:34 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v2] random: set fast pool count to zero in cpuhp teardown

On Mon, Feb 14, 2022 at 3:49 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2022-02-14 15:42:50 [+0100], Jason A. Donenfeld wrote:
> > Hi Sebastian,
> Hi Jason,
>
> > If we move this to startup, is there a phase during which no interrupt
> > will arrive? That is, can this happen very very early in startup, so
> > that zeroing out count happens *before* ++count?
>
> Interrupts will arrive starting with CPUHP_AP_ONLINE_IDLE from the CPU
> HP point of view. My suggestion had a check for upper most bit and only
> clear count if that bit was seen. So we wouldn't clear the counter if we
> wouldn't suspect one of the rare corner cases.

That doesn't work for the other use cases I have for this (see the other patch).

So I think it seems better to keep it before CPUHP_TIMERS_PREPARE, but
do it on startup rather than teardown. Seem reasonable? Would that
mean we zero out before IRQs are enabled?