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

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Message-ID: <e1b80fee-30df-4733-9072-ce67e3edc72f@oracle.com>
Date: Wed, 14 Feb 2024 12:50:31 -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 2/13/2024 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?

>> +       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