KVM: avoid unnecessary synchronize_rcu

We dont have to wait for a grace period if there is no oldpid that
we are going to free. putpid also checks for NULL, so this patch
only fences synchronize_rcu.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 7103f60de8bed21a0ad5d15d2ad5b7a333dda201)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 50f947301fa7abf03f3a7dd870464188e72f84d3..aa5057e7a1b940ef0e64907a51de4aa9d5a5da18 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -129,7 +129,8 @@ int vcpu_load(struct kvm_vcpu *vcpu)
                struct pid *oldpid = vcpu->pid;
                struct pid *newpid = get_task_pid(current, PIDTYPE_PID);
                rcu_assign_pointer(vcpu->pid, newpid);
-               synchronize_rcu();
+               if (oldpid)
+                       synchronize_rcu();
                put_pid(oldpid);
        }
        cpu = get_cpu();