MMIO and gcc re-ordering issue

[Jesse Barnes]

On Tuesday, June 10, 2008 12:05 pm Roland Dreier wrote:
>  > me too.  That's the whole basis for readX_relaxed() and its cohorts: we
>  > make our weirdest machines (like altix) conform to the x86 norm.  Then
>  > where it really kills us we introduce additional semantics to selected
>  > drivers that enable us to recover I/O speed on the abnormal platforms.
>
> Except as I pointed out before, Altix doesn't conform to the norm and
> many (most?) drivers are missing mmiowb()s that are needed for Altix.
> Just no one has plugged most devices into an Altix (or haven't stressed
> the driver in a way that exposes problems of IO ordering between CPUs).
>
> It would be a great thing to use the powerpc trick of setting a flag
> that is tested by spin_unlock()/mutex_unlock() and automatically doing
> the mmiowb() if needed, and then killing off mmiowb() entirely.

Yeah I think that's what Nick's guidelines would guarantee.  And Jes is 
already working on the spin_unlock change you mentioned, so mmiowb() should 
be history soon (in name only, assuming Nick also introduces the I/O barriers 
he talked about for ordering the looser accessors it would still be there but 
would be called io_wmb or something).

Jesse