MMIO and gcc re-ordering issue
paulus at samba.org
Wed May 28 09:04:13 EST 2008
Chris Friesen writes:
> Roland Dreier wrote:
> > 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?
On powerpc we explicitly make sure that can't happen. That's the "do
a sync in spin_unlock if there were any writels since the last
spin_lock" magic. The sync instruction makes sure the writes have got
to the host bridge before the spinlock is unlocked.
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