Re: [RFC -v6 PATCH 7/8] kvm: keep track of which task is running a KVM vcpu

Avi Kivity
2011-01-26T13:02:08Z

On 01/20/2011 11:36 PM, Rik van Riel wrote:
> Keep track of which task is running a KVM vcpu. This helps us
> figure out later what task to wake up if we want to boost a
> vcpu that got preempted.
>
> Unfortunately there are no guarantees that the same task
> always keeps the same vcpu, so we can only track the task
> across a single "run" of the vcpu.
>
> Signed-off-by: Rik van Riel<riel@redhat.com>
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index a055742..9d56ed5 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -81,6 +81,7 @@ struct kvm_vcpu {
> #endif
> int vcpu_id;
> struct mutex mutex;
> + struct pid *pid;
> int cpu;
> atomic_t guest_mode;
> struct kvm_run *run;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5225052..86c4905 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -185,6 +185,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
> vcpu->cpu = -1;
> vcpu->kvm = kvm;
> vcpu->vcpu_id = id;
> + vcpu->pid = NULL;
> init_waitqueue_head(&vcpu->wq);
>
> page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> @@ -208,6 +209,8 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_init);
>
> void kvm_vcpu_uninit(struct kvm_vcpu *vcpu)
> {
> + if (vcpu->pid)
> + put_pid(vcpu->pid);

Unconditional put_pid() suffices.

> kvm_arch_vcpu_uninit(vcpu);
> free_page((unsigned long)vcpu->run);
> }
> @@ -1456,6 +1459,14 @@ static long kvm_vcpu_ioctl(struct file *filp,
> r = -EINVAL;
> if (arg)
> goto out;
> + if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
> + /* The thread running this VCPU changed. */
> + struct pid *oldpid = vcpu->pid;
> + struct pid *newpid = get_task_pid(current, PIDTYPE_PID);
> + rcu_assign_pointer(vcpu->pid, newpid);
> + synchronize_rcu();
> + put_pid(oldpid);
> + }

This is executed without any lock held, so two concurrent KVM_RUNs can
race and cause a double put_pid().

Suggest moving the code to vcpu_load(), where it can execute under the
protection of vcpu->mutex.

--
error compiling committee.c: too many arguments to function