linux-kernel - Re: [PATCH 2/3] s390/vfio-ap: introduce two new r/w locks to replace wait_queue_head

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Date:   Fri, 11 Jun 2021 19:11:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        frankja@...ux.ibm.com, imbrenda@...ux.ibm.com, hca@...ux.ibm.com
Subject: Re: [PATCH 2/3] s390/vfio-ap: introduce two new r/w locks to replace
 wait_queue_head_t

On 11.06.21 19:05, Jason Gunthorpe wrote:
> On Wed, Jun 09, 2021 at 06:46:33PM -0400, Tony Krowiak wrote:
>> This patch introduces two new r/w locks to replace the wait_queue_head_t
>> that was introduced to fix a lockdep splat reported when testing
>> pass-through of AP queues to a Secure Execution guest. This was the
>> abbreviated dependency chain reported by lockdep that was fixed using
>> a wait queue:
>>
>> kvm_arch_crypto_set_masks+0x4a/0x2b8 [kvm]        kvm->lock
>> vfio_ap_mdev_group_notifier+0x154/0x170 [vfio_ap] matrix_dev->lock
>>
>> handle_pqap+0x56/0x1d0 [vfio_ap]    matrix_dev->lock
>> kvm_vcpu_ioctl+0x2cc/0x898 [kvm]    vcpu->mutex
>>
>> kvm_s390_cpus_to_pv+0x4e/0xf8 [kvm]   vcpu->mutex
>> kvm_arch_vm_ioctl+0x3ec/0x550 [kvm]   kvm->lock
> 
> Is the problem larger than kvm_arch_crypto_set_masks()? If not it
> looks easy enough to fix, just pull the kvm->lock out of
> kvm_arch_crypto_set_masks() and obtain it in vfio_ap_mdev_set_kvm()
> before the rwsem. Now your locks are in the right order and all should
> be well?
> 
>> +static int vfio_ap_mdev_matrix_store_lock(struct ap_matrix_mdev *matrix_mdev)
>> +{
>> +	if (!down_write_trylock(&matrix_mdev->rwsem))
>> +		return -EBUSY;
>> +
>> +	if (matrix_mdev->kvm) {
>> +		up_write(&matrix_mdev->rwsem);
>> +		return -EBUSY;
>> +	}
>> +
>> +	if (!down_write_trylock(&matrix_mdev->matrix.rwsem)) {
>> +		up_write(&matrix_mdev->rwsem);
>> +		return -EBUSY;
>> +	}
>> +
>> +	return 0;
>> +}
> 
> This double locking is quite strange, at least it deserves a detailed
> comment? The comments suggest these locks protect distinct data so..
> 
>> +
>> +	ret = vfio_ap_mdev_matrix_store_lock(matrix_mdev);
>> +	if (ret)
>> +		return ret;
>>   
>>   	clear_bit_inv((unsigned long)apqi, matrix_mdev->matrix.aqm);
> 
> here it obtained both locks but only touched matrix.aqm which is only
> protected by the inner lock - what was the point of obtaining the
> outer lock?
> 
> Also, not convinced down_write_trylock() is appropriate from a sysfs
> callback, it should block and wait, surely? Otherwise userspace gets
> random racy failures depending on what the kernel is doing??

It might we worth exploring lock_device_hotplug_sysfs() which does a

"return restart_syscall()" with some delay.


-- 
Thanks,

David / dhildenb