MMIO and gcc re-ordering issue

Roland Dreier rdreier at
Wed May 28 07:29:33 EST 2008

 > > 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.
 > Does that necessarily follow?
 > If you've got a large system with multiple pci bridges, could you end
 > up with posted writes coming from different cpus taking a different
 > amount of time to propagate to a device and thus colliding?

Not on x86.  And a given PCI device can only be reached from a single
host bridge, so I don't see how it can happen.  But on SGI altix
systems, there is a routed fabric between the CPU and the PCI bus, so
the reordering can happen there.  Hence mmiowb() and the endless supply
of driver bugs that it causes.

 - R.

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