RE: [PATCH] vfio: fix deadlock between group lock and kvm lock

[Liu, Yi L]

> From: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> Sent: Wednesday, February 1, 2023 10:43 PM
> 
> On 2/1/23 7:43 AM, Liu, Yi L wrote:
> >> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> >> Sent: Wednesday, February 1, 2023 4:26 AM
> >>
> >> On Tue, Jan 31, 2023 at 03:06:35PM -0500, Matthew Rosato wrote:
> >>> @@ -799,13 +794,14 @@
> >> EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
> >>>  void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
> >>>  {
> >>>  	struct vfio_group *group = file->private_data;
> >>> +	unsigned long flags;
> >>>
> >>>  	if (!vfio_file_is_group(file))
> >>>  		return;
> >>>
> >>> -	mutex_lock(&group->group_lock);
> >>> +	spin_lock_irqsave(&group->kvm_ref_lock, flags);
> >>>  	group->kvm = kvm;
> >>> -	mutex_unlock(&group->group_lock);
> >>> +	spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
> >>
> >> We know we are in a sleeping context here so these are just
> >> 'spin_lock()', same with the other one
> >
> > a dumb question. Why spinlock is required here? 😊
> >
> 
> You mean as opposed to another mutex?  I don't think it's required per se
> (we are replacing a mutex so we could have again used another mutex
> here), but all current users of this new lock hold it over a very short window
> (e.g. set a pointer as above, or refcount++ and copy the pointer as in the
> first device_open)

I see. Just not sure if spinlock is required for a special reason.

Regards,
Yi Liu