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

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Message-ID: <CAJaqyWdKC6rLBOykSeWc87QtJ757LNXd78R59JRizUyAQCu+ig@mail.gmail.com>
Date: Thu, 15 Feb 2024 16:44:30 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: Steven Sistare <steven.sistare@...cle.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 Wed, Feb 14, 2024 at 8:52 PM Steven Sistare
<steven.sistare@...cle.com> wrote:
>
> 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.
>

But it is ok to have concurrent mm updates, isn't it? Moreover, they
can be enqueued immediately after the kthread_flush_work already, as
there is no lock protecting it.

> - 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
> >>
> >
>