MMIO and gcc re-ordering issue

Linus Torvalds torvalds at
Wed May 28 07:55:56 EST 2008

On Wed, 28 May 2008, Benjamin Herrenschmidt wrote:
> A problem with __raw_ though is that they -also- don't do byteswap,

Well, that's why there is __readl() and __raw_readl(), no?

Neither does ordering, and __raw_readl() doesn't do byte-swap.

Of course, I'm not going to guarantee every architecture even has all 
those versions, nor am I going to guarantee they all work as advertised :)

For x86, they have historially all been 100% identical. With the inline 
asm patch I posted, the "__" version (whether "raw" or not) lack the 
"memory" barrier, so they allow a *little* bit more re-ordering.

(They won't be re-ordered wrt spinlocks etc, unless gcc starts reordering 
volatile asm's against each other, which would be a bug).

In practice, I doubt it matters. Whatever small compiler re-ordering it 
might affect won't have any real performance impack one way or the other, 
I think.


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