linux-kernel - Re: [PATCH v3] hwrng: core: Freeze khwrng thread during suspend

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Date:   Tue, 29 Oct 2019 00:45:31 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Andrey Pronin <apronin@...omium.org>,
        Duncan Laurie <dlaurie@...omium.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guenter Roeck <groeck@...omium.org>,
        Alexander Steffen <Alexander.Steffen@...ineon.com>
Subject: Re: [PATCH v3] hwrng: core: Freeze khwrng thread during suspend

Hi Stephen,

On 06.08.2019 01:32, Stephen Boyd wrote:
> The hwrng_fill() function can run while devices are suspending and
> resuming. If the hwrng is behind a bus such as i2c or SPI and that bus
> is suspended, the hwrng may hang the bus while attempting to add some
> randomness. It's been observed on ChromeOS devices with suspend-to-idle
> (s2idle) and an i2c based hwrng that this kthread may run and ask the
> hwrng device for randomness before the i2c bus has been resumed.
> 
> Let's make this kthread freezable so that we don't try to touch the
> hwrng during suspend/resume. This ensures that we can't cause the hwrng
> backing driver to get into a bad state because the device is guaranteed
> to be resumed before the hwrng kthread is thawed.

This patch broke suspend with virtio-rng loaded (it hangs).

The problematic call chain is:
virtrng_freeze() -> remove_common() -> hwrng_unregister() ->
kthread_stop().

It looks like kthread_stop() can't finish on a frozen khwrng thread.

Reverting this commit makes a VM with virtio-rng driver loaded
suspend and resume correctly again.

Maciej