netdev - Re: [PATCH]: 8139cp: Don't blindly enable interrupts in cp_start

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Date:	Tue, 16 Jan 2007 09:44:06 -0500
From:	Chris Lalancette <clalance@...hat.com>
To:	Francois Romieu <romieu@...zoreil.com>
CC:	jgarzik@...ox.com, netdev@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH]: 8139cp: Don't blindly enable interrupts in cp_start_xmit

Francois Romieu wrote:
> Chris Lalancette <clalance@...hat.com> :
> [...]
> 
>>     Similar to this commit:
>>
>>http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=d15e9c4d9a75702b30e00cdf95c71c88e3f3f51e
>>
>>It's not safe in cp_start_xmit to blindly call spin_lock_irq and then
>>spin_unlock_irq, since it may very well be the case that cp_start_xmit
>>was called with interrupts already disabled (I came across this bug in
>>the context of netdump in RedHat kernels, but the same issue holds, for
>>example, in netconsole).  Therefore, replace all instances of spin_lock_irq
>>and spin_unlock_irq with spin_lock_irqsave and spin_unlock_irqrestore,
>>respectively, in cp_start_xmit().  I tested this against a fully-virtualized
>>Xen guest, which happens to use the 8139cp driver to talk to the emulated
>>hardware.  I don't have a real piece of 8139cp hardware to test on, so
>>someone else will have to do that.
> 
> 
> (message reformated to fit in 80 columns, please fix your mailer)
> 
> As I understand http://lkml.org/lkml/2006/12/12/239, something like the
> patch below should had been sent instead. Herbert, ack/nak ?
> 

Francois,
     Thanks for the comments.  While the patch you sent will help, there are
still other places that will have problems.  For example, in netpoll_send_skb,
we call local_irq_save(flags), then call dev->hard_start_xmit(), and then call
local_irq_restore(flags).  This is a similar situation to what I described
above; we will re-enable interrupts in cp_start_xmit(), when netpoll_send_skb
doesn't expect that, and will probably run into issues.
     Is there a problem with changing cp_start_xmit to use the
spin_lock_irqsave(), besides the extra instructions it needs?

Thanks,
Chris Lalancette
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