netdev - Re: crash with bridge and inconsistent handling of NETDEV_TX

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Message-Id: <20100420.164809.78193162.davem@davemloft.net>
Date:	Tue, 20 Apr 2010 16:48:09 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mpatocka@...hat.com
Cc:	lists.linux-foundation.org@...hat.com, netdev@...r.kernel.org,
	kaber@...sh.net
Subject: Re: crash with bridge and inconsistent handling of NETDEV_TX_OK

From: David Miller <davem@...emloft.net>
Date: Tue, 20 Apr 2010 16:45:05 -0700 (PDT)

> From: Mikulas Patocka <mpatocka@...hat.com>
> Date: Tue, 20 Apr 2010 19:40:56 -0400 (EDT)
> 
>> Reviewing the code further, I found one very weird commit
>> 572a9d7b6fc7f20f573664063324c086be310c42 committed to 2.6.33. What
>> it does, it changes the semantics of ndo_hard_start_xmit(). Prior to
>> the patch, the meaning was --- return zero (NETDEV_TX_OK) --- the
>> skb is consumed by the driver. Returns non-zero --- the skb is left
>> owned by the caller.  The patch changes it to return other flags in
>> bits 4-7 and changes the consumed/returned logic.
>> 
>> The problem is that there is still plenty of code that compares it
>> against NETDEV_TX_OK to find out if the skb was consumed.
> 
> Drivers are not supposed to return those new flag bits, the new flag
> bits as return values exist only in the packet scheduler path.

And BTW, NETDEV_TX_OK is only ever returned by itself, the
flag bits only get set when a non-NETDEV_TX_OK value is returned.

So we really haven't changed semantics at all, NETDEV_TX_OK (which is
zero) and non-zero are the two valid return value cases.
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