netdev - Re: [PATCH 2/3] NET: [CORE] Stack changes to add multiqueue hardware support API

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Date:	Thu, 28 Jun 2007 21:20:04 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org, jeff@...zik.org,
	"Kok, Auke-jan H" <auke-jan.h.kok@...el.com>, hadi@...erus.ca
Subject: Re: [PATCH 2/3] NET: [CORE] Stack changes to add multiqueue hardware
 support API

Waskiewicz Jr, Peter P wrote:
>>Waskiewicz Jr, Peter P wrote:
>>
>>Yes, I noticed that now. Doesn't seem right though as long as
>>queueing while queue is stopped is treated as a bug by the
>>drivers.
>>
>>But I vaguely recall seeing a discussion about this, I'll check
>>the archives.
> 
> 
> The basic gist is before the dequeue is done, the qdisc is locked by the
> qdisc is running bit, so another CPU cannot get in there.  So if the
> queue isn't stopped when a dequeue is done, that same queue should not
> be stopped when hard_start_xmit() is called.  The only thing I could
> think of that could happen is some out-of-band cleanup routine in the
> driver where the tx_ring lock is held, and the skb is bounced back,
> where the driver returns NETIF_TX_BUSY, and you requeue.  This is an
> extreme corner case, so the check could be removed.


Yes, but there are users that don't go through qdiscs, like netpoll,
Having them check the QDISC_RUNNING bit seems ugly.
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