linux-kernel - Re: [PATCH] virtio: Remove virtio device during shutdown

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Date:	Thu, 12 Mar 2015 17:39:31 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>, Fam Zheng <famz@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org,
	Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH] virtio: Remove virtio device during shutdown



On 12/03/2015 17:22, Michael S. Tsirkin wrote:
> On Wed, Mar 11, 2015 at 06:11:35PM +0800, Fam Zheng wrote:
>> On Wed, 03/11 10:06, Michael S. Tsirkin wrote:
>>> On Wed, Mar 11, 2015 at 04:09:17PM +0800, Fam Zheng wrote:
>>>> Currently shutdown is nop for virtio devices, but the core code could
>>>> remove things behind us such as MSI-X handler etc. For example in the
>>>> case of virtio-scsi-pci, the device may still try to send interupts,
>>>> which will be on IRQ lines seeing MSI-X disabled. Those interrupts will
>>>> be unhandled, and may cause flood.
>>
>> Here is the problem I want to solve - file system driver hang:
>>
>> If a fs code happen to hit __wait_on_buffer right after pci pci_device_shutdown
>> disabled msix, it will never make progress because the requests it waits for
>> will never be completed. So the system hangs.
> 
> Paolo says that pci reset of virtio scsi device guarantees
> that all outstanding requests complete.

For what it's worth, see here:

static void virtio_scsi_reset(VirtIODevice *vdev)
{
    ...
    s->resetting++;
    qbus_reset_all(&s->bus.qbus);
    s->resetting--;
    ...
}

static void scsi_disk_reset(DeviceState *dev)
{
    SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev.qdev, dev);
    uint64_t nb_sectors;

    scsi_device_purge_requests(&s->qdev, SENSE_CODE(RESET));
    ...
}

Paolo

> If true and implemented correctly, I don't see what else
> needs to be done.
> 
> You will need to debug this some more.
> 
> 
>> In other words we will want to reset virtio device before pci_device_shutdown
>> AND wake up all waiters.
>>
>> Unfortunately, neither your patch nor mine does that, because virtio bus can be
>> shutdown after pci bus (thanks to Jason for pointing out this). In that case,
>> any completion after disabling msix is lost.
>>
>> Maybe we need both the pci shutdown handler to reset the device and the virtio
>> shutdown handler to remove the device?
>>
>> Fam
>>
>>>
>>> This sounds very tentative. Do you, in fact, observe some problems
>>> with virtio scsi? How to reproduce them? this needs to go
>>> into the commit messages.
>>
>> OK, my bad.
>>
>>>
>>>> Remove the device in "shutdown" callback to allow device drivers clean
>>>> up things.
>>>>
>>>> Signed-off-by: Fam Zheng <famz@...hat.com>
>>>
>>> I'm concerned this will cause more hangs on shutdown: one
>>> of the reasons for reboot is device mal-functioning.
>>> How about we just reset devices instead? Something like
>>> the below (untested).
>>>
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>>>
>>> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
>>> index 5ce2aa4..0769941 100644
>>> --- a/drivers/virtio/virtio.c
>>> +++ b/drivers/virtio/virtio.c
>>> @@ -269,6 +269,17 @@ static int virtio_dev_remove(struct device *_d)
>>>  	return 0;
>>>  }
>>>  
>>> +static void virtio_dev_shutdown(struct device *_d)
>>> +{
>>> +	struct virtio_device *dev = dev_to_virtio(_d);
>>> +	/*
>>> +	 * Reset the device to make it stop sending interrupts, DMA, etc.
>>> +	 * We are shutting down, no need for full cleanup.
>>> +	 */
>>> +	dev->config->reset(dev);
>>> +
>>> +}
>>> +
>>>  static struct bus_type virtio_bus = {
>>>  	.name  = "virtio",
>>>  	.match = virtio_dev_match,
>>> @@ -276,6 +288,7 @@ static struct bus_type virtio_bus = {
>>>  	.uevent = virtio_uevent,
>>>  	.probe = virtio_dev_probe,
>>>  	.remove = virtio_dev_remove,
>>> +	.shutdown = virtio_dev_shutdown,
>>>  };
>>>  
>>>  bool virtio_device_is_legacy_only(struct virtio_device_id id)
--
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