Subject: Re: [PATCH net 3/4] xen-netback: Fix releasing header slot on error path

From: Zoltan Kiss <[email-masked]>
Date: Fri, 18 Jul 2014 19:06:41 +0100

On 18/07/14 16:25, Wei Liu wrote:
> On Thu, Jul 17, 2014 at 08:09:51PM +0100, Zoltan Kiss wrote:
>> This patch makes this function aware that the first frag and the header might
>> share the same ring slot. That could happen if the first slot is bigger than
>> MAX_SKB_LEN. Due to this the error path might release that slot twice or never,
>
> I guess you mean PKT_PROT_LEN.
Yes
>
> Just one question, how come that we didn't come across this with copying
> backend? Comparing txreq.size against PKT_PROT_LEN is not new in mapping
> backend.
We had one grant copy for the header and the first frag in that case, 
and we skipped the first frag:

	/* Skip first skb fragment if it is on same page as header fragment. */
	start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);

>
>> depending on the error scenario.
>> xenvif_idx_release is also removed from xenvif_idx_unmap, and called separately.
>>
>> Signed-off-by: Zoltan Kiss <[email-masked]>
>> Reported-by: Armin Zentai <[email-masked]>
>> Cc: [email-masked]
>> Cc: [email-masked]
>> Cc: [email-masked]
>> ---
>> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
>> index e9ffb05..938d5a9 100644
>> --- a/drivers/net/xen-netback/netback.c
>> +++ b/drivers/net/xen-netback/netback.c
>> @@ -1039,6 +1039,8 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
>>   	 */
>>   	struct skb_shared_info *first_shinfo = NULL;
>>   	int nr_frags = shinfo->nr_frags;
>> +	const bool sharedslot = nr_frags &&
>> +				frag_get_pending_idx(&shinfo->frags[0]) == pending_idx;
>>   	int i, err;
>>
>>   	/* Check status of header. */
>> @@ -1051,7 +1053,10 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
>>   				   (*gopp_copy)->status,
>>   				   pending_idx,
>>   				   (*gopp_copy)->source.u.ref);
>> -		xenvif_idx_release(queue, pending_idx, XEN_NETIF_RSP_ERROR);
>> +		/* The first frag might still have this slot mapped */
>> +		if (!sharedslot)
>> +			xenvif_idx_release(queue, pending_idx,
>> +					   XEN_NETIF_RSP_ERROR);
>>   	}
>>
>>   check_frags:
>> @@ -1068,8 +1073,19 @@ check_frags:
>>   						pending_idx,
>>   						gop_map->handle);
>>   			/* Had a previous error? Invalidate this fragment. */
>> -			if (unlikely(err))
>> +			if (unlikely(err)) {
>>   				xenvif_idx_unmap(queue, pending_idx);
>> +				/* If the mapping of the first frag was OK, but
>> +				 * the header's copy failed, and they are
>> +				 * sharing a slot, send an error
>> +				 */
>> +				if (i == 0 && sharedslot)
>> +					xenvif_idx_release(queue, pending_idx,
>> +							   XEN_NETIF_RSP_ERROR);
>> +				else
>> +					xenvif_idx_release(queue, pending_idx,
>> +							   XEN_NETIF_RSP_OKAY);
>
> I guess this is sort of OK, just a bit fragile. Couldn't think of a
> better way to refactor this function. :-(
I was thinking a lot about how to refactor this whole thing, but I gave 
up too ...
>
>> +			}
>>   			continue;
>>   		}
>>
>> @@ -1081,15 +1097,27 @@ check_frags:
>>   				   gop_map->status,
>>   				   pending_idx,
>>   				   gop_map->ref);
>> +
>
> Stray blank line.
>
> And did you miss a callsite of xenvif_idx_unmap in this function which
> is added in your first patch?
Nope, the xenvif_idx_release is there
> Wei.