Messages in this thread | | | From | Roland Dreier <> | Subject | Re: MMIO and gcc re-ordering issue | Date | Tue, 27 May 2008 11:07:05 -0700 |
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> 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.
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