netdev - Re: [PATCH resend 1/3] Revert "net: fec: fix missing napi

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Message-ID: <20130428093917.GA4540@electric-eye.fr.zoreil.com>
Date:	Sun, 28 Apr 2013 11:39:17 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Frank Li <lznuaa@...il.com>
Cc:	Robert Schwebel <r.schwebel@...gutronix.de>,
	David Miller <davem@...emloft.net>,
	Lucas Stach <l.stach@...gutronix.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	Fabio Estevam <festevam@...il.com>,
	Shawn Guo <shawn.guo@...aro.org>
Subject: Re: [PATCH resend 1/3] Revert "net: fec: fix missing napi_disable
 call"

Frank Li <lznuaa@...il.com> :
[...]
> This is lockless design if only one read and one write. It likes kfifo.
> 
> Assume CPU1 run xmit.  CPU2 run napi fec_enet_tx
> 
> (1)       if (fep->cur_tx == fep->dirty_tx)
> (2)                 netif_stop_queue(ndev);
> 
> if CPU2 update fep->dirty_tx before CPU1 run (1).  the condition is false.
> if CPU2 update fep->dirty_tx after (1) before (2),   netif_stop_queue
> will be called.  There are not problem at this time even though queue
> is not full here.  queue is not empty for sure here. fec_enet_tx will
> be called again when NAPI trigger by one frame finished transfer,
> fec_enet_tx will wake up send queue.

"before" and "after" may not work as expected between different CPU without
explicit synchronization (or barrier). It won't oops but it would be careful
to envision something like drivers/net/ethernet/broadcom/tg3.c::tg3_tx.

-- 
Ueimor
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