netdev - Re: [PATCH] ixgbe: Introduce new 10GbE driver for Intel 82598 based PCI Express adapters...

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Date:	Thu, 05 Jul 2007 19:29:01 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Inaky Perez-Gonzalez <inaky@...ux.intel.com>
CC:	"Kok, Auke" <auke-jan.h.kok@...el.com>,
	Michael Buesch <mb@...sch.de>,
	Christoph Hellwig <hch@...radead.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	"Veeraiyan, Ayyappan" <ayyappan.veeraiyan@...el.com>,
	netdev@...r.kernel.org, arjan@...ux.intel.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] ixgbe: Introduce new 10GbE driver for Intel 82598 based
 PCI Express adapters...

Inaky Perez-Gonzalez wrote:
> On Tuesday 03 July 2007, Jeff Garzik wrote:
>> Inaky Perez-Gonzalez wrote:
>> Access to bitfields are not atomic within the machine int in which they 
>> are stored... you need to "unpack" the values stored in bitfields, even 
>> if they are single-bit bitfields.
> 
> Which we do manually when we don't use bitfields. Again, conceptually,
> there is no difference.

Practically speaking -- there are differences.

The "manual" method hides nothing from the programmer, while use of 
bitfields hides the lack of atomicity.

When you have programmers who make mistakes -- i.e. real humans -- these 
things matter.

But overall, it is not any one detail that discourages use of bitfields; 
it is the sum of all the reasons.  Practical experience, compiler 
technology, mistakes made (and not made), all point to avoiding bitfields.

	Jeff


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