linux-kernel - Re: [RFC -v5 PATCH 1/4] kvm: keep track of which task is running a KVM vcpu

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Message-ID: <4D330C1F.1000600@redhat.com>
Date:	Sun, 16 Jan 2011 17:17:51 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rik van Riel <riel@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Chris Wright <chrisw@...s-sol.org>, ttracy@...hat.com,
	dshaks@...hat.com
Subject: Re: [RFC -v5 PATCH 1/4] kvm: keep track of which task is running
 a KVM vcpu

On 01/14/2011 10:03 AM, 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.
>
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5225052..65e997a 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 = 0;

NULL

> @@ -1456,6 +1459,12 @@ 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;
> +			vcpu->pid = get_task_pid(current, PIDTYPE_PID);
> +			put_pid(oldpid);
> +		}

This is subject to the same race as before.  If another vcpu picks up 
vcpu->pid before the assignment (that is, oldpid), but dereferences it 
after put_pid(), it hits freed memory.

You want something like

     struct pid *oldpid = vcpu->pid;
     rcu_assign_pointer(vcpu->pid, get_task_pid());
     synchronize_rcu();
     put_pid(oldpid);

with rcu_read_lock() / rcu_dereference() protection on the reader side.

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

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