Subject: Re: [PATCH kernel v2 2/6] KVM: PPC: Use RCU for arch.spapr_tce_tables

From: David Gibson <[email-masked]>
Date: Mon, 25 Jan 2016 10:46:44 +1100

On Thu, Jan 21, 2016 at 06:39:33PM +1100, Alexey Kardashevskiy wrote:
> At the moment spapr_tce_tables is not protected against races.

That's not really true - it's protected by the kvm->lock mutex.

> This makes
> use of RCU-variants of list helpers. As some bits are executed in real
> mode, this makes use of just introduced list_for_each_entry_rcu_notrace().
> 
> This converts release_spapr_tce_table() to a RCU scheduled handler.

The change itself is fine, though.

> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> Reviewed-by: David Gibson <[email-masked]>
> ---
>  arch/powerpc/include/asm/kvm_host.h |  1 +
>  arch/powerpc/kvm/book3s.c           |  2 +-
>  arch/powerpc/kvm/book3s_64_vio.c    | 20 +++++++++++---------
>  3 files changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> index 271fefb..c7ee696 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -184,6 +184,7 @@ struct kvmppc_spapr_tce_table {
>  	struct kvm *kvm;
>  	u64 liobn;
>  	u32 window_size;
> +	struct rcu_head rcu;
>  	struct page *pages[0];
>  };
>  
> diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> index 638c6d9..b34220d 100644
> --- a/arch/powerpc/kvm/book3s.c
> +++ b/arch/powerpc/kvm/book3s.c
> @@ -807,7 +807,7 @@ int kvmppc_core_init_vm(struct kvm *kvm)
>  {
>  
>  #ifdef CONFIG_PPC64
> -	INIT_LIST_HEAD(&kvm->arch.spapr_tce_tables);
> +	INIT_LIST_HEAD_RCU(&kvm->arch.spapr_tce_tables);
>  	INIT_LIST_HEAD(&kvm->arch.rtas_tokens);
>  #endif
>  
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 54cf9bc..9526c34 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -45,19 +45,16 @@ static long kvmppc_stt_npages(unsigned long window_size)
>  		     * sizeof(u64), PAGE_SIZE) / PAGE_SIZE;
>  }
>  
> -static void release_spapr_tce_table(struct kvmppc_spapr_tce_table *stt)
> +static void release_spapr_tce_table(struct rcu_head *head)
>  {
> -	struct kvm *kvm = stt->kvm;
> +	struct kvmppc_spapr_tce_table *stt = container_of(head,
> +			struct kvmppc_spapr_tce_table, rcu);
>  	int i;
>  
> -	mutex_lock(&kvm->lock);
> -	list_del(&stt->list);
>  	for (i = 0; i < kvmppc_stt_npages(stt->window_size); i++)
>  		__free_page(stt->pages[i]);
> +
>  	kfree(stt);
> -	mutex_unlock(&kvm->lock);
> -
> -	kvm_put_kvm(kvm);
>  }
>  
>  static int kvm_spapr_tce_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> @@ -88,7 +85,12 @@ static int kvm_spapr_tce_release(struct inode *inode, struct file *filp)
>  {
>  	struct kvmppc_spapr_tce_table *stt = filp->private_data;
>  
> -	release_spapr_tce_table(stt);
> +	list_del_rcu(&stt->list);
> +
> +	kvm_put_kvm(stt->kvm);
> +
> +	call_rcu(&stt->rcu, release_spapr_tce_table);
> +
>  	return 0;
>  }
>  
> @@ -131,7 +133,7 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  	kvm_get_kvm(kvm);
>  
>  	mutex_lock(&kvm->lock);
> -	list_add(&stt->list, &kvm->arch.spapr_tce_tables);
> +	list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
>  
>  	mutex_unlock(&kvm->lock);
>  

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson