Re: MMIO and gcc re-ordering issue

Pavel Machek
2008-06-01T14:45:05Z

Hi!

> > Though it's my understanding that at least ia64 does require the
> > explicit barriers anyway, so we are still in a dodgy situation here
> > where it's not clear what drivers should do and we end up with
> > possibly excessive barriers on powerpc where I end up with both
> > the wmb/rmb/mb that were added for ia64 -and- the ones I have in
> > readl/writel to make them look synchronous... Not nice.
>
> ia64 is a disaster with a slightly different ordering problem -- the
> mmiowb() issue. I know Ben knows far too much about this, but for big
> SGI boxes, you sometimes need mmiowb() to avoid problems with driver
> code that does totally sane stuff like
>
> spin_lock(&mmio_lock);
> writel(val1, reg1);
> writel(val2, reg2);
> spin_unlock(&mmio_lock);
>
> If that snippet is called on two CPUs at the same time, then the device
> might see a sequence like
>
> CPU1 -- write reg1
> CPU2 -- write reg1
> CPU1 -- write reg2
> CPU2 -- write reg2
>
> in spite of the fact that everything is totally ordered on the CPUs by
> the spin lock.
>
> The reason this is such a disaster is because the code looks right,
> makes sense, and works fine on 99.99% of all systems out there. So I
> would bet that 99% of our drivers have missing mmiowb() "bugs" -- no one
> has plugged the hardware into an Altix box and cared enough to stress
> test it.

Yes, ia64 is a disaster, and needs its spinlock implementation fixed
:-).
Pavel
--
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