Re: [Xen-devel] network hang trigger

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> in file net/core.c
> ======================================
> if (!dev->hard_start_xmit(skb, dev)) {
>         HARD_TX_UNLOCK_BH(dev);
>         goto out;
> }
> ...
> 
> out_kfree_skb:
>         kfree_skb(skb);
> out:
>         return rc;
> ======================================
> 
> Bingo, it doesn't. And take a look at other network driver source codes, 
> e.g. 8139too.c, 3c501.c etc, they all ***free skb*** upon error and 
> ***always return 0***. This is *hidden contract* between the caller and the 
> callee.

(1) See drivers like 3c59x.c. They do not free the skbuff when they
    return non-zero.

(2) Look again at the code fragment you've posted -- the body of the
    'if' statement is executed when hard_start_xmit returns zero. So
    the body corresponds to the NON-ERROR case! You'll see that the
    error case does in fact free the skbuff.

> So, skbuffs don't get freed until gc'ed.

No, there is no bug here. And if we were leaking skbuffs they would
never be freed. There is no automatic garbage collection within the
kernel.

 -- Keir


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