linux-kernel - Re: [PATCH v4 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks

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Date:   Thu, 18 Mar 2021 13:54:06 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, stable@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com, kwankhede@...dia.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        pasic@...ux.vnet.ibm.com
Subject: Re: [PATCH v4 1/1] s390/vfio-ap: fix circular lockdep when
 setting/clearing crypto masks

On 3/17/21 7:17 PM, Halil Pasic wrote:
> On Wed, 10 Mar 2021 10:05:59 -0500
> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
>
>> -		ret = vfio_ap_mdev_reset_queues(mdev);
>> +		matrix_mdev = mdev_get_drvdata(mdev);
> Is it guaranteed that matrix_mdev can't be NULL here? If yes, please
> remind me of the mechanism that ensures this.

The matrix_mdev is set as drvdata when the mdev is created and
is only cleared when the mdev is removed. Likewise, this function
is a callback defined by by vfio in the vfio_ap_matrix_ops structure
when the matrix_dev is registered and is intended to handle ioctl
calls from userspace during the lifetime of the mdev. While I can't
speak definitively to the guarantee, I think it is extremely unlikely
that matrix_mdev would be NULL at this point. On the other hand,
it wouldn't hurt to check for NULL and log an error or warning
message (I prefer an error here) if NULL.

>
>> +
>> +		/*
>> +		 * If the KVM pointer is in the process of being set, wait until
>> +		 * the process has completed.
>> +		 */
>> +		wait_event_cmd(matrix_mdev->wait_for_kvm,
>> +			       matrix_mdev->kvm_busy == false,
>> +			       mutex_unlock(&matrix_dev->lock),
>> +			       mutex_lock(&matrix_dev->lock));
>> +
>> +		if (matrix_mdev->kvm)
>> +			ret = vfio_ap_mdev_reset_queues(mdev);
>> +		else
>> +			ret = -ENODEV;
> Didn't we agree to make the call to vfio_ap_mdev_reset_queues()
> unconditional again (for reference please take look at
> Message-ID: <64afa72c-2d6a-2ca1-e576-34e15fa579ed@...ux.ibm.com>)?

Yes, we did agree to that and I changed it at the time. That change
got lost somehow; I'll reinstate it.

>
> Regards,
> Halil