Messages in this thread | | | Date | Fri, 7 Nov 2008 19:55:25 -0500 (EST) | From | Steven Rostedt <> | Subject | Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb() |
| |
On Fri, 7 Nov 2008, David Howells wrote:
> Steven Rostedt <rostedt@goodmis.org> wrote: > > > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could > > > be required so it works also on UP systems safely wrt interrupts). > > > > smp_rmb turns into a compiler barrier on UP and should prevent the below > > description. > > Note that that does not guarantee that the two reads will be done in the order > you want. The compiler barrier _only_ affects the compiler. It does not stop > the CPU from doing the reads in any order it wants. You need something > stronger than smp_rmb() if you need the reads to be so ordered.
For reading hardware devices that can indeed be correct. But for normal memory access on a uniprocessor, if the CPU were to reorder the reads that would effect the actual algorithm then that CPU is broken.
read a <--- interrupt - should see read a here before read b is done. read b
Now the fact that one of the reads is a hardware clock, then this statement might not be too strong. But the fact that it is a clock, and not some memory mapped device register, I still think smp_rmb is sufficient.
-- Steve
| |