netdev - Re: [PATCH] xen-netback: correct success/error reporting for the SKB-with-fraglist case

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Date:   Thu, 25 Feb 2021 15:00:56 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     paul@....org
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Wei Liu <wl@....org>
Subject: Re: [PATCH] xen-netback: correct success/error reporting for the
 SKB-with-fraglist case

On 25.02.2021 13:11, Paul Durrant wrote:
> On 25/02/2021 07:33, Jan Beulich wrote:
>> On 24.02.2021 17:39, Paul Durrant wrote:
>>> On 23/02/2021 16:29, Jan Beulich wrote:
>>>> When re-entering the main loop of xenvif_tx_check_gop() a 2nd time, the
>>>> special considerations for the head of the SKB no longer apply. Don't
>>>> mistakenly report ERROR to the frontend for the first entry in the list,
>>>> even if - from all I can tell - this shouldn't matter much as the overall
>>>> transmit will need to be considered failed anyway.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>>>>
>>>> --- a/drivers/net/xen-netback/netback.c
>>>> +++ b/drivers/net/xen-netback/netback.c
>>>> @@ -499,7 +499,7 @@ check_frags:
>>>>    				 * the header's copy failed, and they are
>>>>    				 * sharing a slot, send an error
>>>>    				 */
>>>> -				if (i == 0 && sharedslot)
>>>> +				if (i == 0 && !first_shinfo && sharedslot)
>>>>    					xenvif_idx_release(queue, pending_idx,
>>>>    							   XEN_NETIF_RSP_ERROR);
>>>>    				else
>>>>
>>>
>>> I think this will DTRT, but to my mind it would make more sense to clear
>>> 'sharedslot' before the 'goto check_frags' at the bottom of the function.
>>
>> That was my initial idea as well, but
>> - I think it is for a reason that the variable is "const".
>> - There is another use of it which would then instead need further
>>    amending (and which I believe is at least part of the reason for
>>    the variable to be "const").
>>
> 
> Oh, yes. But now that I look again, don't you want:
> 
> if (i == 0 && first_shinfo && sharedslot)
> 
> ? (i.e no '!')
> 
> The comment states that the error should be indicated when the first 
> frag contains the header in the case that the map succeeded but the 
> prior copy from the same ref failed. This can only possibly be the case 
> if this is the 'first_shinfo'

I don't think so, no - there's a difference between "first frag"
(at which point first_shinfo is NULL) and first frag list entry
(at which point first_shinfo is non-NULL).

> (which is why I still think it is safe to unconst 'sharedslot' and
> clear it).

And "no" here as well - this piece of code

		/* First error: if the header haven't shared a slot with the
		 * first frag, release it as well.
		 */
		if (!sharedslot)
			xenvif_idx_release(queue,
					   XENVIF_TX_CB(skb)->pending_idx,
					   XEN_NETIF_RSP_OKAY);

specifically requires sharedslot to have the value that was
assigned to it at the start of the function (this property
doesn't go away when switching from fragments to frag list).
Note also how it uses XENVIF_TX_CB(skb)->pending_idx, i.e. the
value the local variable pending_idx was set from at the start
of the function.

Jan