Assume the time source is a global clock which insures that time will never *ever* go backward. Use a smp_rmb() to make sure the cnt_lo value is read before __m_cnt_hi. Remove the now unnecessary volatile statement. The barrier takes care of memory ordering. Mathieu: > Yup, you are right. However, the case where one CPU sees the clock source > a little bit off-sync (late) still poses a problem. Example follows : > > CPU A B > read __m_cnt_hi (0x80000000) > read hw cnt low (0x00000001) > (wrap detected : > (s32)(0x80000000 ^ 0x1) < 0) > write __m_cnt_hi = 0x00000001 > return 0x0000000100000001 > read __m_cnt_hi (0x00000001) > (late) read hw cnt low (0xFFFFFFFA) > (wrap detected : > (s32)(0x00000001 ^ 0xFFFFFFFA) < +0) > write __m_cnt_hi = 0x80000001 > return 0x80000001FFFFFFFA > (time jumps) > A similar situation can be generated by out-of-order hi/low bits reads. Nicolas: This, of course, should and can be prevented. No big deal. Signed-off-by: Mathieu Desnoyers CC: Nicolas Pitre CC: Ralf Baechle CC: benh@kernel.crashing.org CC: paulus@samba.org CC: David Miller CC: Linus Torvalds CC: Andrew Morton CC: Ingo Molnar CC: Peter Zijlstra CC: Thomas Gleixner CC: Steven Rostedt CC: linux-arch@vger.kernel.org --- include/linux/cnt32_to_63.h | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) Index: linux-2.6-lttng/include/linux/cnt32_to_63.h =================================================================== --- linux-2.6-lttng.orig/include/linux/cnt32_to_63.h 2008-11-04 01:39:03.000000000 -0500 +++ linux-2.6-lttng/include/linux/cnt32_to_63.h 2008-11-04 01:48:50.000000000 -0500 @@ -65,12 +65,17 @@ union cnt32_to_63 { * implicitly by making the multiplier even, therefore saving on a runtime * clear-bit instruction. Otherwise caller must remember to clear the top * bit explicitly. + * + * Assume the time source is a global clock read from memory mapped I/O which + * insures that time will never *ever* go backward. Using a smp_rmb() to make + * sure the __m_cnt_hi value is read before the cnt_lo mmio read. */ #define cnt32_to_63(cnt_lo) \ ({ \ - static volatile u32 __m_cnt_hi; \ + static u32 __m_cnt_hi; \ union cnt32_to_63 __x; \ __x.hi = __m_cnt_hi; \ + smp_rmb(); /* read __m_cnt_hi before mmio cnt_lo */ \ __x.lo = (cnt_lo); \ if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \ __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \ -- 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-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/