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

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Date:	Sun, 28 Apr 2013 11:09:48 +0800
From:	Frank Li <lznuaa@...il.com>
To:	Francois Romieu <romieu@...zoreil.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"

2013/4/28 Francois Romieu <romieu@...zoreil.com>:
> Frank Li <lznuaa@...il.com> :
>> 2013/4/27, Francois Romieu <romieu@...zoreil.com>:
> [...]
>> > napi_disable may sleep.
>> >
>> > fec_restart can be called with spinlock held in fec_enet_adjust_link
>>
>> You are right. Spin lock in FEC enet adjust link can be removed when
>> remove spinlock in tx and RX.
>
> fec_restart is also called from the netdev watchdog handler (fec_timeout)
> and tasklet can't sleep either. You should imho schedule_work from
> fec_timeout.

I will add delay_work to fix this issue.

>
> How is the driver supposed to avoid the napi context
> fec_enet_tx:netif_queue_stopped vs fec_enet_start_xmit:netif_stop_queue
> race since both can run on different cpu without any read/write ordering
> enforcement between one thread netif_* and its peer {dirty/cur}_tx ?

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.

Most worse case is waste one BD script.

>
> --
> Ueimor
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