netdev - Re: ixgbe: Set IXGBE_RSC_CB(skb)->DMA field to zero after unmapping the address

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Message-ID: <1269387653.8599.132.camel@pasglop>
Date:	Wed, 24 Mar 2010 10:40:53 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Malli <mchilakala@...il.com>
Cc:	Mallikarjuna R Chilakala <mallikarjuna.chilakala@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: ixgbe: Set IXGBE_RSC_CB(skb)->DMA field to zero after
 unmapping the address

On Tue, 2010-03-23 at 10:40 -0700, Malli wrote:
> 
> Yes. I just realized that i can't assign a zero magic "bad" value. It
> is only
> valid in x86/arm/m68k/alpha architecures and not in spark & PowerPC
> arch,
> (it should be ~0). In some other architecutres it throws a BUG() on
> with
> dma_mapping_error() checks. So the patch is not a total bogus in some
> architectures :(.
> 
> May be it is best to create an internal FLAG in IXGBE_RSC_CB(skb)
> which can be used to avoid this double freeing. 

I suppose if Linus reads that he'll just come back and say "you idiots,
just make 0 invalid everywhere" like he did for interrupts tho :-) It
wouldn't be too hard for us to make the IOMMU allocator always skip "0".

But for now, I'd suggest you add a flag.

Cheers,
Ben.


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