On Sun, 28 Aug 2011, Ming Lei wrote:
> Looks like there is still another similar problem in qh_link_async():
> the last wmb
> should be changed into mb, because HC will read 'head->hw->hw_next' from qh
> descriptor and this pointer in qh is read only for HC. But this problem can't
> be
> observed on ARM, since wmb on ARM is same with mb.
It doesn't matter what the HC does -- the wmb() instruction is executed
by the CPU, not the HC. The point of that instruction is to make sure
that the
qh->hw->hw_next = head->hw->hw_next;
line (and all the preceding lines as well) is ordered before the
head->hw->hw_next = dma;
3C
line. Since both of these lines are writes, not reads, it suffices to
use wmb() rather than mb().
Alan Stern
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