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

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 18 Mar 2021 13:54:06 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> > 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. 

Yes, I've checked that these are all callbacks in the same struct, so
the callbacks are all registered simultaneously, i.e. the ioctl callback
gettin gregistered only when drv_data is already set is not the case.
If there isn't a mechanism in core mdev, then I think we better be
careful.  I don't see what would guarantee the pointer is always in the
vfio_ap code. 

> 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 we aren't absolutely sure this pointer is going to be always a valid
one, let's check it!

Regards,
Halil