Re: MMIO and gcc re-ordering issue

Roland Dreier
2008-05-27T18:07:07Z

> Um, OK, you've said write twice now ... I was assuming you meant read.
> Even on an x86, writes are posted, so there's no way a spin lock could
> serialise a write without an intervening read to flush the posting
> (that's why only reads have a relaxed version on altix). Or is there
> something else I'm missing?

Writes are posted yes, but not reordered arbitrarily. If I have code like:

spin_lock(&mmio_lock);
writel(val1, reg1);
writel(val2, reg2);
spin_unlock(&mmio_lock);

then I have a reasonable expectation that if two CPUs run this at the
same time, their writes to reg1/reg2 won't be interleaved with each
other (because the whole section is inside a spinlock). And Altix
violates that expectation.

- R.