Re: [PATCH 1/2] vhost-user: fix lost reconnect

[Raphael Norwitz]

> On Aug 22, 2023, at 12:49 AM, Li Feng <fengli@smartx.com> wrote:
> 
> 
> 
>> On 22 Aug 2023, at 8:38 AM, Raphael Norwitz <raphael.norwitz@nutanix.com> 
>> wrote:
>> 
>>> 
>>> On Aug 17, 2023, at 2:40 AM, Li Feng <fengli@smartx.com> wrote:
>>> 
>>> 
>>>> 2023年8月14日 下午8:11，Raphael Norwitz <raphael.norwitz@nutanix.com> 写道：
>>>> 
>>>> Why can’t we rather fix this by adding a “event_cb” param to 
>>>> vhost_user_async_close and then call qemu_chr_fe_set_handlers in 
>>>> vhost_user_async_close_bh()?
>>>> 
>>>> Even if calling vhost_dev_cleanup() twice is safe today I worry future 
>>>> changes may easily stumble over the reconnect case and introduce crashes 
>>>> or double frees.
>>>> 
>>> I think add a new event_cb is not good enough. ‘qemu_chr_fe_set_handlers’ 
>>> has been called in vhost_user_async_close, and will be called in event->cb, 
>>> so why need add a
>>> new event_cb?
>>> 
>> 
>> I’m suggesting calling the data->event_cb instead of the data->cb if we hit 
>> the error case where vhost->vdev is NULL. Something like:
>> 
>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>> index 8dcf049d42..edf1dccd44 100644
>> --- a/hw/virtio/vhost-user.c
>> +++ b/hw/virtio/vhost-user.c
>> @@ -2648,6 +2648,10 @@ typedef struct {
>> static void vhost_user_async_close_bh(void *opaque)
>> {
>>     VhostAsyncCallback *data = opaque;
>> +
>> +    VirtIODevice *vdev = VIRTIO_DEVICE(data->dev);
>> +    VHostUserBlk *s = VHOST_USER_BLK(vdev);
>> +
>>     struct vhost_dev *vhost = data->vhost;
>> 
>>     /*
>> @@ -2657,6 +2661,9 @@ static void vhost_user_async_close_bh(void *opaque)
>>      */
>>     if (vhost->vdev) {
>>         data->cb(data->dev);
>> +    } else if (data->event_cb) {
>> +        qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, data->event_cb,
>> +                                 NULL, data->dev, NULL, true);
>>     }
>> 
>>     g_free(data);
>> 
>> data->event_cb would be vhost_user_blk_event().
>> 
>> I think that makes the error path a lot easier to reason about and more 
>> future proof.
>> 
>>> For avoiding to call the vhost_dev_cleanup() twice, add a ‘inited’ in 
>>> struct vhost-dev to mark if it’s inited like this:
>>> 
>> 
>> This is better than the original, but let me know what you think of my 
>> alternative.
> 
> The vhost_user_async_close_bh() is a common function in vhost-user.c, and 
> vhost_user_async_close() is used by vhost-user-scsi/blk/gpio, 
> However, in your patch it’s limited to VhostUserBlk, so I think my fix is 
> more reasonable.

I did not write out the full patch. 

vhost-user-scsi/blk/gpio would each provide their own ->event_cb, just like 
they each provide a different ->cb today. Looking at it again, data has the 
chardev, so no need to use VIRTO_DEVICE() or VHOST_USER_BLK().

The fix generalizes to all device types.

> 
> Thanks,
> LI
>> 
>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>> index e2f6ffb446..edc80c0231 100644
>>> --- a/hw/virtio/vhost.c
>>> +++ b/hw/virtio/vhost.c
>>> @@ -1502,6 +1502,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void 
>>> *opaque,
>>>        goto fail_busyloop;
>>>    }
>>> 
>>> +    hdev->inited = true;
>>>    return 0;
>>> 
>>> fail_busyloop:
>>> @@ -1520,6 +1521,10 @@ void vhost_dev_cleanup(struct vhost_dev *hdev)
>>> {
>>>    int i;
>>> 
>>> +    if (!hdev->inited) {
>>> +        return;
>>> +    }
>>> +    hdev->inited = false;
>>>    trace_vhost_dev_cleanup(hdev);
>>> 
>>>    for (i = 0; i < hdev->nvqs; ++i) {
>>> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
>>> index ca3131b1af..74b1aec960 100644
>>> --- a/include/hw/virtio/vhost.h
>>> +++ b/include/hw/virtio/vhost.h
>>> @@ -123,6 +123,7 @@ struct vhost_dev {
>>>    /* @started: is the vhost device started? */
>>>    bool started;
>>>    bool log_enabled;
>>> +    bool inited;
>>>    uint64_t log_size;
>>>    Error *migration_blocker;
>>>    const VhostOps *vhost_ops;
>>> 
>>> Thanks.
>>> 
>>>> 
>>>>> On Aug 4, 2023, at 1:29 AM, Li Feng <fengli@smartx.com> wrote:
>>>>> 
>>>>> When the vhost-user is reconnecting to the backend, and if the vhost-user 
>>>>> fails
>>>>> at the get_features in vhost_dev_init(), then the reconnect will fail
>>>>> and it will not be retriggered forever.
>>>>> 
>>>>> The reason is:
>>>>> When the vhost-user fail at get_features, the vhost_dev_cleanup will be 
>>>>> called
>>>>> immediately.
>>>>> 
>>>>> vhost_dev_cleanup calls 'memset(hdev, 0, sizeof(struct vhost_dev))'.
>>>>> 
>>>>> The reconnect path is:
>>>>> vhost_user_blk_event
>>>>> vhost_user_async_close(.. vhost_user_blk_disconnect ..)
>>>>>  qemu_chr_fe_set_handlers <----- clear the notifier callback
>>>>>    schedule vhost_user_async_close_bh
>>>>> 
>>>>> The vhost->vdev is null, so the vhost_user_blk_disconnect will not be
>>>>> called, then the event fd callback will not be reinstalled.
>>>>> 
>>>>> With this patch, the vhost_user_blk_disconnect will call the
>>>>> vhost_dev_cleanup() again, it's safe.
>>>>> 
>>>>> All vhost-user devices have this issue, including vhost-user-blk/scsi.
>>>>> 
>>>>> Fixes: 71e076a07d ("hw/virtio: generalise CHR_EVENT_CLOSED handling")
>>>>> 
>>>>> Signed-off-by: Li Feng <fengli@smartx.com>
>>>>> ---
>>>>> hw/virtio/vhost-user.c | 10 +---------
>>>>> 1 file changed, 1 insertion(+), 9 deletions(-)
>>>>> 
>>>>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>>>>> index 8dcf049d42..697b403fe2 100644
>>>>> --- a/hw/virtio/vhost-user.c
>>>>> +++ b/hw/virtio/vhost-user.c
>>>>> @@ -2648,16 +2648,8 @@ typedef struct {
>>>>> static void vhost_user_async_close_bh(void *opaque)
>>>>> {
>>>>>  VhostAsyncCallback *data = opaque;
>>>>> -    struct vhost_dev *vhost = data->vhost;
>>>>> 
>>>>> -    /*
>>>>> -     * If the vhost_dev has been cleared in the meantime there is
>>>>> -     * nothing left to do as some other path has completed the
>>>>> -     * cleanup.
>>>>> -     */
>>>>> -    if (vhost->vdev) {
>>>>> -        data->cb(data->dev);
>>>>> -    }
>>>>> +    data->cb(data->dev);
>>>>> 
>>>>>  g_free(data);
>>>>> }
>>>>> -- 
>>>>> 2.41.0