linux-kernel - Re: [PATCH V2 3/3] vdpa_sim: flush workers on suspend

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Date: Wed, 14 Feb 2024 14:52:17 -0500
From: Steven Sistare <steven.sistare@...cle.com>
To: Eugenio Perez Martin <eperezma@...hat.com>
Cc: virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, Si-Wei Liu <si-wei.liu@...cle.com>,
        Xie Yongji <xieyongji@...edance.com>,
        Stefano Garzarella <sgarzare@...hat.com>
Subject: Re: [PATCH V2 3/3] vdpa_sim: flush workers on suspend

On 2/14/2024 2:39 PM, Eugenio Perez Martin wrote:
> On Wed, Feb 14, 2024 at 6:50 PM Steven Sistare
> <steven.sistare@...cle.com> wrote:
>>
>> On 2/13/2024 11:10 AM, Eugenio Perez Martin wrote:
>>> On Mon, Feb 12, 2024 at 6:16 PM Steve Sistare <steven.sistare@...cle.com> wrote:
>>>>
>>>> Flush to guarantee no workers are running when suspend returns.
>>>>
>>>> Signed-off-by: Steve Sistare <steven.sistare@...cle.com>
>>>> ---
>>>>  drivers/vdpa/vdpa_sim/vdpa_sim.c | 13 +++++++++++++
>>>>  1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
>>>> index be2925d0d283..a662b90357c3 100644
>>>> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
>>>> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
>>>> @@ -74,6 +74,17 @@ static void vdpasim_worker_change_mm_sync(struct vdpasim *vdpasim,
>>>>         kthread_flush_work(work);
>>>>  }
>>>>
>>>> +static void flush_work_fn(struct kthread_work *work) {}
>>>> +
>>>> +static void vdpasim_flush_work(struct vdpasim *vdpasim)
>>>> +{
>>>> +       struct kthread_work work;
>>>> +
>>>> +       kthread_init_work(&work, flush_work_fn);
>>>
>>> If the work is already queued, doesn't it break the linked list
>>> because of the memset in kthread_init_work?
>>
>> work is a local variable.  It completes before vdpasim_flush_work returns,
>> thus is never already queued on entry to vdpasim_flush_work.
>> Am I missing your point?
> 
> No, sorry, I was the one missing that. Thanks for explaining it :)!
> 
> I'm not so used to the kthread queue, but why not calling
> kthread_flush_work on vdpasim->work directly?

vdpasim->work is not the only work posted to vdpasim->worker; see 
vdpasim_worker_change_mm_sync.  Posting a new no-op work guarantees
they are all flushed.

- Steve

>>>> +       kthread_queue_work(vdpasim->worker, &work);
>>>> +       kthread_flush_work(&work);
>>>> +}
>>>> +
>>>>  static struct vdpasim *vdpa_to_sim(struct vdpa_device *vdpa)
>>>>  {
>>>>         return container_of(vdpa, struct vdpasim, vdpa);
>>>> @@ -511,6 +522,8 @@ static int vdpasim_suspend(struct vdpa_device *vdpa)
>>>>         vdpasim->running = false;
>>>>         mutex_unlock(&vdpasim->mutex);
>>>>
>>>> +       vdpasim_flush_work(vdpasim);
>>>
>>> Do we need to protect the case where vdpasim_kick_vq and
>>> vdpasim_suspend are called "at the same time"? Correct userland should
>>> not be doing it but buggy or mailious could be. Just calling
>>> vdpasim_flush_work with the mutex acquired would solve the issue,
>>> doesn't it?
>>
>> Good catch.  I need to serialize access to vdpasim->running plus the worker queue
>> in these two functions.  vdpasim_kick_vq currently takes no locks. In case it is called
>> from non-task contexts, I should define a new spinlock to be acquired in both functions.
>>
>> - Steve
>>
>