KVM: s390: pv: simplify shutdown and fix race

Simplify the shutdown of non-protected VMs. There is no need to do
complex manipulations of the counter if it was zero.

This also fixes a very rare race which caused pages to be torn down
from the address space with a non-zero counter even on older machines
that don't support the UVC instruction, causing a crash.

Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Fixes: fb491d5500 ("KVM: s390: pv: asynchronous destroy for reboot")
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20230705111937.33472-2-imbrenda@linux.ibm.com>

arch/s390/kvm/pv.c

@@ -411,8 +411,12 @@ int kvm_s390_pv_deinit_cleanup_all(struct kvm *kvm, u16 *rc, u16 *rrc)
 	u16 _rc, _rrc;
 	int cc = 0;
 
-	/* Make sure the counter does not reach 0 before calling s390_uv_destroy_range */
-	atomic_inc(&kvm->mm->context.protected_count);
+	/*
+	 * Nothing to do if the counter was already 0. Otherwise make sure
+	 * the counter does not reach 0 before calling s390_uv_destroy_range.
+	 */
+	if (!atomic_inc_not_zero(&kvm->mm->context.protected_count))
+		return 0;
 
 	*rc = 1;
 	/* If the current VM is protected, destroy it */