* Steven Rostedt (rostedt@xxxxxxxxxxx) wrote: > > On Fri, 7 Nov 2008, Mathieu Desnoyers wrote: > > > > I want to make sure > > > > __m_cnt_hi > > is read before > > mmio cnt_lo read > > Hmm, let me make sure I understand why there is no wmb. > > Paul, can you verify this? > > Mathieu, you do the following: > > read a > smp_rmb > reab b > if (test b) > write a > > So the idea is that you must read b to test it. And since we must read a > before reading b we can see that we write a before either? > > The question remains, can the write happen before either of the reads? > write a cannot happen before read a (same variable). write a must happen after read b because it depends on the b value. It makes sure the the side-effect of "write a" is seen by other CPUs *after* we have read the b value. > But since the read b is reading the hw clock, perhaps that just implies a > wmb on the hardware side? > It makes sense. The hardware clock has no cache coherency problem.. so it could be seen as doing wmb() after each data update. Mathieu > -- Steve -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
