On 3/25/21 4:32 PM, Halil Pasic wrote:
On Thu, 25 Mar 2021 08:46:40 -0400 Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:This patch fixes a lockdep splat introduced by commit f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated"). The lockdep splat only occurs when starting a Secure Execution guest. Crypto virtualization (vfio_ap) is not yet supported for SE guests; however, in order to avoid this problem when support becomes available, this fix is being provided. The circular locking dependency was introduced when the setting of the masks in the guest's APCB was executed while holding the matrix_dev->lock. While the lock is definitely needed to protect the setting/unsetting of the matrix_mdev->kvm pointer, it is not necessarily critical for setting the masks; so, the matrix_dev->lock will be released while the masks are being set or cleared. Keep in mind, however, that another process that takes the matrix_dev->lock can get control while the masks in the guest's APCB are being set or cleared as a result of the driver being notified that the KVM pointer has been set or unset. This could result in invalid access to the matrix_mdev->kvm pointer by the intervening process. To avoid this scenario, two new fields are being added to the ap_matrix_mdev struct: struct ap_matrix_mdev { ... bool kvm_busy; wait_queue_head_t wait_for_kvm; ... }; The functions that handle notification that the KVM pointer value has been set or cleared will set the kvm_busy flag to true until they are done processing at which time they will set it to false and wake up the tasks on the matrix_mdev->wait_for_kvm wait queue. Functions that require access to matrix_mdev->kvm will sleep on the wait queue until they are awakened at which time they can safely access the matrix_mdev->kvm field. Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated") Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx> I intend to give a couple of work-days to others, and if nobody objects merge this. (I will wait till Tuesday.)
Thanks Halil.
I've tested it and it does silence the lockdep splat. Regards, Halil
