Message-ID: <9b6c0c4d-57c9-6a9a-379e-82f073b73988@linux.ibm.com>
Date:   Wed, 1 Feb 2023 09:27:45 -0500
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Subject: Re: [PATCH] vfio: fix deadlock between group lock and kvm lock
To:     "Tian, Kevin" <kevin.tian@intel.com>,
        "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>, "jgg@nvidia.com" <jgg@nvidia.com>
Cc:     "cohuck@redhat.com" <cohuck@redhat.com>,
        "farman@linux.ibm.com" <farman@linux.ibm.com>,
        "pmorel@linux.ibm.com" <pmorel@linux.ibm.com>,
        "borntraeger@linux.ibm.com" <borntraeger@linux.ibm.com>,
        "frankja@linux.ibm.com" <frankja@linux.ibm.com>,
        "imbrenda@linux.ibm.com" <imbrenda@linux.ibm.com>,
        "david@redhat.com" <david@redhat.com>,
        "akrowiak@linux.ibm.com" <akrowiak@linux.ibm.com>,
        "jjherne@linux.ibm.com" <jjherne@linux.ibm.com>,
        "pasic@linux.ibm.com" <pasic@linux.ibm.com>,
        "zhenyuw@linux.intel.com" <zhenyuw@linux.intel.com>,
        "Wang, Zhi A" <zhi.a.wang@intel.com>,
        "Christopherson, , Sean" <seanjc@google.com>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "intel-gvt-dev@lists.freedesktop.org" 
        <intel-gvt-dev@lists.freedesktop.org>,
        "intel-gfx@lists.freedesktop.org" <intel-gfx@lists.freedesktop.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20230131200635.44227-1-mjrosato@linux.ibm.com>
 <BN9PR11MB5276E39EE37A7BA2E1B896278CD19@BN9PR11MB5276.namprd11.prod.outlook.com>
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From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <BN9PR11MB5276E39EE37A7BA2E1B896278CD19@BN9PR11MB5276.namprd11.prod.outlook.com>
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On 2/1/23 1:13 AM, Tian, Kevin wrote:
>> From: Matthew Rosato <mjrosato@linux.ibm.com>
>> Sent: Wednesday, February 1, 2023 4:07 AM
>>
>> -	device->kvm = kvm;
>> +	/*
>> +	 * Get the KVM pointer currently associated with the group, if there
>> +	 * is one, and obtain a reference now that will be held until the
>> +	 * open_count reaches 0.  Save the pointer in the device for use by
>> +	 * drivers.
>> +	 */
>> +	spin_lock_irqsave(&group->kvm_ref_lock, flags);
>> +	if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
>> +		device->kvm = group->kvm;
>> +	spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
>> +
> 
> No group reference in vfio_main.c. 

OK -- I think to do that I'll have to move the lock/unlock of the dev_set lock to group.c right before we call vfio_device_{open,close} and check open_count there to make decisions about the kvm ref (before calling vfio_device_open to decide to get kvm ref, after returning from vfio_device_open to see if we to drop ref on error, after close to see if we need to drop ref).

> 
> btw Yi, looks the deadlock issue also exists in your cdev work.
> 
> kvm_vfio_release() holds kvm lock and then try to acquire
> device->device_set->lock in vfio_device_file_set_kvm().
> 
> vfio_device_ioctl_bind_iommufd() holds device->device_set->lock
> and then call vfio_device_open() which finally hit kvm lock
> acquisition in driver's open_device routine (e.g. vfio-ap).
> 
> A similar fix is required in your series.
> 
> Thanks
> Kevin