netdev - Re: [PATCH] stmmac: call local_irq_save other than disable_irq in poll

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Date:	Wed, 6 May 2015 11:53:53 +0800
From:	Sonic Zhang <sonic.adi@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Giuseppe CAVALLARO <peppe.cavallaro@...com>,
	netdev <netdev@...r.kernel.org>,
	adi-buildroot-devel@...ts.sourceforge.net,
	"Zhang, Sonic" <sonic.zhang@...log.com>
Subject: Re: [PATCH] stmmac: call local_irq_save other than disable_irq in poll_controller

Hi David,

On Wed, May 6, 2015 at 7:36 AM, David Miller <davem@...emloft.net> wrote:
> From: Sonic Zhang <sonic.adi@...il.com>
> Date: Tue, 5 May 2015 15:55:53 +0800
>
>> From: Sonic Zhang <sonic.zhang@...log.com>
>>
>> It is possible that poll_controller is called from netpoll.c with irq
>> already disabled. Unexpected interrupt may be triggered if always
>> enable irq before returning from poll_controller.
>>
>> Signed-off-by: Sonic Zhang <sonic.zhang@...log.com>
>
> Nope, this is wrong.
>
> If you don't use disable_irq(), the interrupt handler can run in
> parallel on another cpu and you definitely do not want that.

How about change local_irq_save(flags) to
spin_lock_irqsave(&priv->lock, flags) ?

>
> There is nothing wrong with the interrupt triggering on the
> enable_irq() here.  If that causes some problem, there is a
> bug in this driver.

When I debug Linux kernel via KGDB over Ethernet, KGDB calls netpoll
API and stmmac_poll_controller to send UDP packets with all interrupts
disabled. KGDB assumes all interrupts keeps disabled when it is active
and doing UDP data transmission. Enabling interrupts in
stmmac_poll_controller causes KGDB re-enter exception and system halt.

Although KGDB over Ethernet is not part of the mainline tree, I feel
this patch is useful in case other kernel functions want to call
netpoll API with interrupts disabled.

Regards,

Sonic
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