[PATCH v1 1/8] KVM: s390: vsie: Avoid potential deadlock with real spaces

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

The natural lock ordering is mmu_lock -> children_lock, but in
gmap_create_shadow() the reverse order is used when handling shadowing
of real address spaces.

Convert the inner locking of kvm->mmu_lock to a trylock; return -EAGAIN
if the lock is busy, and let the caller try again.

This path is not expected to happen in real-life scenarios, so its
performance is not important.

Fixes: a2c17f9270cc ("KVM: s390: New gmap code")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/gmap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index 298fbaecec28..8abb4f55b306 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -1374,8 +1374,13 @@ struct gmap *gmap_create_shadow(struct kvm_s390_mmu_cache *mc, struct gmap *pare
 			/* Only allow one real-space gmap shadow. */
 			list_for_each_entry(sg, &parent->children, list) {
 				if (sg->guest_asce.r) {
-					scoped_guard(write_lock, &parent->kvm->mmu_lock)
+					if (write_trylock(&parent->kvm->mmu_lock)) {
 						gmap_unshadow(sg);
+						write_unlock(&parent->kvm->mmu_lock);
+					} else {
+						gmap_put(new);
+						return ERR_PTR(-EAGAIN);
+					}
 					break;
 				}
 			}
-- 
2.55.0