linux-kernel - Re: [PATCH v3] vfio: fix potential deadlock on vfio group lock

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Date:   Fri, 13 Jan 2023 14:52:56 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     alex.williamson@...hat.com, pbonzini@...hat.com, cohuck@...hat.com,
        farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, frankja@...ux.ibm.com,
        imbrenda@...ux.ibm.com, david@...hat.com, akrowiak@...ux.ibm.com,
        jjherne@...ux.ibm.com, pasic@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com, seanjc@...gle.com,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] vfio: fix potential deadlock on vfio group lock

On Fri, Jan 13, 2023 at 12:11:32PM -0500, Matthew Rosato wrote:
> @@ -462,9 +520,19 @@ static inline void vfio_device_pm_runtime_put(struct vfio_device *device)
>  static int vfio_device_fops_release(struct inode *inode, struct file *filep)
>  {
>  	struct vfio_device *device = filep->private_data;
> +	struct kvm *kvm = NULL;
>  
>  	vfio_device_group_close(device);
>  
> +	mutex_lock(&device->dev_set->lock);
> +	if (device->open_count == 0 && device->kvm) {
> +		kvm = device->kvm;
> +		device->kvm = NULL;
> +	}
> +	mutex_unlock(&device->dev_set->lock);

This still doesn't seem right, another thread could have incr'd the
open_count already 

This has to be done at the moment open_count is decremented to zero,
while still under the original lock.

Jason