linux-kernel - Re: [PATCH v7 1/7] vduse: make domain

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Message-ID: <CAJaqyWeeJVM6VeYp4HUEe0ZZAi26Jgqw8WaEg6POoGcUOkk6Qw@mail.gmail.com>
Date: Thu, 23 Oct 2025 10:59:01 +0200
From: Eugenio Perez Martin <eperezma@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Yongji Xie <xieyongji@...edance.com>, Stefano Garzarella <sgarzare@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, virtualization@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, jasowang@...hat.com, 
	Maxime Coquelin <mcoqueli@...hat.com>, Laurent Vivier <lvivier@...hat.com>, Cindy Lu <lulu@...hat.com>
Subject: Re: [PATCH v7 1/7] vduse: make domain_lock an rwlock

On Fri, Oct 10, 2025 at 2:34 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>

...

>
>
> > @@ -2045,11 +2046,11 @@ static int vdpa_dev_add(struct vdpa_mgmt_dev *mdev, const char *name,
> >       if (ret)
> >               return ret;
> >
> > -     mutex_lock(&dev->domain_lock);
> > +     write_lock(&dev->domain_lock);
> >       if (!dev->domain)
> >               dev->domain = vduse_domain_create(VDUSE_IOVA_SIZE - 1,
> >                                                 dev->bounce_size);
> > -     mutex_unlock(&dev->domain_lock);
> > +     write_unlock(&dev->domain_lock);
> >       if (!dev->domain) {
> >               put_device(&dev->vdev->vdpa.dev);
> >               return -ENOMEM;
>
>
> Let's look at this example:
>
> So now you are invoking this under an rw lock:
>
>
>
>
> struct vduse_iova_domain *
> vduse_domain_create(unsigned long iova_limit, size_t bounce_size)
> {
>         struct vduse_iova_domain *domain;
>         struct file *file;
>         struct vduse_bounce_map *map;
>         unsigned long pfn, bounce_pfns;
>         int ret;
>
>         bounce_pfns = PAGE_ALIGN(bounce_size) >> BOUNCE_MAP_SHIFT;
>         if (iova_limit <= bounce_size)
>                 return NULL;
>
>         domain = kzalloc(sizeof(*domain), GFP_KERNEL);
>         if (!domain)
>                 return NULL;
>
>
> ...
>
>
>
> Which unless I am mistaken will produce a lockdep splat and deadlock.
>

Can you expand on this? All that code works with and without lockdep.

>
> So it looks like the previous version did not compile
> and this one looks DOA.  What's up?
>

In case it is useful, I forgot to enable some vendor vdpa drivers of
HW I don't have, and that was the reason why it didn't compile. But I
didn't send it without testing it of course.

> At this stage please include information about configs you
> tested, and how.
>
> And any locking changes should also be tested with lockdep enabled
> please.
>

This is the text I'm planning to include in the next series, please
let me know if you want me to expand:

Tested by creating a VDUSE device OVS with and without MQ, and live
migrating between two hosts back and forth while maintaining ping
alive in all the stages. All tested with and without lockdep.