On Fri, 2 Sep 2011, Ming Lei wrote: > On Wed, Aug 31, 2011 at 11:05 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > You might want to remove also the first wmb() in qh_append_tds(). > > I think the first wmb() is used to order the two writing: > > qtd->hw_token = HALT_BIT(ehci); > wmb () > *dummy = *qtd; > > Since dummy has been in hw queue already and must see halt bit of hw_token > first, the barrier should be kept, otherwise the transaction represented by qtd > may be executed too early and qtd->hw_next is still not ready. The transaction represented by qtd can't be executed before the transaction represented by dummy, since qtd gets spliced at the end of the transaction list (it becomes the new dummy). The transaction represented by dummy won't be executed until after dummy->hw_token is set to token, and there's already a memory barrier before that instruction. This memory barrier will insure that qtd->hw_token is ready in time. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
