linux-kernel - Re: [PATCH v5 2/9] ALSA: virtio: add virtio sound driver

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Message-ID: <cea4164d-345e-393b-9328-731c2f874eb7@opensynergy.com>
Date:   Thu, 25 Feb 2021 12:51:36 +0100
From:   Anton Yakovlev <anton.yakovlev@...nsynergy.com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     <virtualization@...ts.linux-foundation.org>,
        <alsa-devel@...a-project.org>, <virtio-dev@...ts.oasis-open.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Jason Wang <jasowang@...hat.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/9] ALSA: virtio: add virtio sound driver

On 25.02.2021 11:38, Takashi Iwai wrote:
> On Mon, 22 Feb 2021 16:34:37 +0100,
> Anton Yakovlev wrote:
>> +static int virtsnd_find_vqs(struct virtio_snd *snd)
>> +{
>> +     struct virtio_device *vdev = snd->vdev;
>> +     vq_callback_t *callbacks[VIRTIO_SND_VQ_MAX] = {
>> +             [VIRTIO_SND_VQ_EVENT] = virtsnd_event_notify_cb
>> +     };
>> +     const char *names[VIRTIO_SND_VQ_MAX] = {
> 
> Shouldn't be static?

Well, yes. Although in this particular case, I do not think it is that
critical. :)


> Also it's often const char * const names[] = { ... }
> unless you overwrite something.

I tried to use the same type names as in the function prototype.
Otherwise the compiler or static analyzer may complain.


>> +/**
>> + * virtsnd_reset_fn() - Kernel worker's function to reset the device.
>> + * @work: Reset device work.
>> + *
>> + * Context: Process context.
>> + */
>> +static void virtsnd_reset_fn(struct work_struct *work)
>> +{
>> +     struct virtio_snd *snd =
>> +             container_of(work, struct virtio_snd, reset_work);
>> +     struct virtio_device *vdev = snd->vdev;
>> +     struct device *dev = &vdev->dev;
>> +     int rc;
>> +
>> +     dev_info(dev, "sound device needs reset\n");
>> +
>> +     /*
>> +      * It seems that the only way to properly reset the device is to remove
>> +      * and re-create the ALSA sound card device.
>> +      */
>> +     rc = device_reprobe(dev);
>> +     if (rc)
>> +             dev_err(dev, "failed to reprobe sound device: %d\n", rc);
> 
> Now I'm wondering whether it's safe to do that from this place.
> Basically device_reprobe() unbinds the device that releases the full
> resources once including the devm_* stuff.  And this work itself is in
> a part of devm allocated resource, so it'll be released there.  That
> said, we might hit use-after-free...  This needs to be verified.

It's safe. Suicide kernel workers are funny but possible things. Since
the kernel itself (AFAIU) assumes such a situation and does not access
the worker structure after the callback function call.


> thanks,
> 
> Takashi
> 

-- 
Anton Yakovlev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin