linux-kernel - Re: [PULL 2/2] vhost: replace rcu with mutex

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Message-ID: <1401746280.3645.187.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 02 Jun 2014 14:58:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org
Subject: Re: [PULL 2/2] vhost: replace rcu with mutex

On Tue, 2014-06-03 at 00:30 +0300, Michael S. Tsirkin wrote:
> All memory accesses are done under some VQ mutex.
> So lock/unlock all VQs is a faster equivalent of synchronize_rcu()
> for memory access changes.
> Some guests cause a lot of these changes, so it's helpful
> to make them faster.
> 
> Reported-by: "Gonglei (Arei)" <arei.gonglei@...wei.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/vhost/vhost.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 78987e4..1c05e60 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -593,6 +593,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
>  {
>  	struct vhost_memory mem, *newmem, *oldmem;
>  	unsigned long size = offsetof(struct vhost_memory, regions);
> +	int i;
>  
>  	if (copy_from_user(&mem, m, size))
>  		return -EFAULT;
> @@ -619,7 +620,14 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
>  	oldmem = rcu_dereference_protected(d->memory,
>  					   lockdep_is_held(&d->mutex));
>  	rcu_assign_pointer(d->memory, newmem);
> -	synchronize_rcu();
> +
> +	/* All memory accesses are done under some VQ mutex.
> +	 * So below is a faster equivalent of synchronize_rcu()
> +	 */
> +	for (i = 0; i < d->nvqs; ++i) {
> +		mutex_lock(&d->vqs[i]->mutex);
> +		mutex_unlock(&d->vqs[i]->mutex);
> +	}
>  	kfree(oldmem);
>  	return 0;
>  }

This looks dubious

What about using kfree_rcu() instead ?

translate_desc() still uses rcu_read_lock(), its not clear if the mutex
is really held.



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