linux-kernel - Re: [PATCH] UML - Correctly handle skb allocation failures

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Date:	Thu, 27 Sep 2007 16:53:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Dike <jdike@...toit.com>
Cc:	linux-kernel@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH] UML - Correctly handle skb allocation failures

On Thu, 27 Sep 2007 13:01:26 -0400
Jeff Dike <jdike@...toit.com> wrote:

> +static int update_drop_skb(int max)
> +{
> +	struct sk_buff *new;
> +	int err = 0;
> +
> +	spin_lock(&drop_lock);
> +
> +	if (max <= drop_max)
> +		goto out;
> +
> +	err = -ENOMEM;
> +	new = dev_alloc_skb(max);
> +	if (new == NULL)
> +		goto out;
> +
> +	skb_put(new, max);
> +
> +	kfree_skb(drop_skb);
> +	drop_skb = new;
> +	drop_max = max;
> +	err = 0;
> +out:
> +	spin_unlock(&drop_lock);
> +
> +	return err;
> +}
> +
>  static int uml_net_rx(struct net_device *dev)
>  {
>  	struct uml_net_private *lp = dev->priv;
> @@ -43,6 +82,9 @@ static int uml_net_rx(struct net_device 
>  	/* If we can't allocate memory, try again next round. */
>  	skb = dev_alloc_skb(lp->max_packet);
>  	if (skb == NULL) {
> +		drop_skb->dev = dev;
> +		/* Read a packet into drop_skb and don't do anything with it. */
> +		(*lp->read)(lp->fd, drop_skb, lp);
>  		lp->stats.rx_dropped++;
>  		return 0;

Still wanna know why it is safe for uml_net_rx to be playing with
drop_skb when update_drop_skb() could be concurrently reallocating
and freeing it.
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