linux-kernel - Re: [PATCH] vfio: fix deadlock between group lock and kvm lock

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Date:   Wed, 1 Feb 2023 09:42:49 -0500
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>, Jason Gunthorpe <jgg@...dia.com>
Cc:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "farman@...ux.ibm.com" <farman@...ux.ibm.com>,
        "pmorel@...ux.ibm.com" <pmorel@...ux.ibm.com>,
        "borntraeger@...ux.ibm.com" <borntraeger@...ux.ibm.com>,
        "frankja@...ux.ibm.com" <frankja@...ux.ibm.com>,
        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>,
        "david@...hat.com" <david@...hat.com>,
        "akrowiak@...ux.ibm.com" <akrowiak@...ux.ibm.com>,
        "jjherne@...ux.ibm.com" <jjherne@...ux.ibm.com>,
        "pasic@...ux.ibm.com" <pasic@...ux.ibm.com>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>,
        "Wang, Zhi A" <zhi.a.wang@...el.com>,
        "Christopherson, , Sean" <seanjc@...gle.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio: fix deadlock between group lock and kvm lock

On 2/1/23 7:43 AM, Liu, Yi L wrote:
>> From: Jason Gunthorpe <jgg@...dia.com>
>> 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)