clarify usage expectations for cnt32_to_63()

Currently, all existing users of cnt32_to_63() are fine since the CPU
architectures where it is used don't do read access reordering, and user
mode preemption is disabled already.  It is nevertheless a good idea to
better elaborate usage requirements wrt preemption, and use an explicit
memory barrier on SMP to avoid different CPUs accessing the counter
value in the wrong order.  On UP a simple compiler barrier is
sufficient.
Signed-off-by: Nicolas Pitre <nico@marvell.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cnt32_to_63.h

@@ -16,6 +16,7 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <asm/byteorder.h>
+#include <asm/system.h>
 
 /* this is used only to give gcc a clue about good code generation */
 union cnt32_to_63 {
@@ -53,11 +54,19 @@ union cnt32_to_63 {
  * needed increment.  And any race in updating the value in memory is harmless
  * as the same value would simply be stored more than once.
  *
- * The only restriction for the algorithm to work properly is that this
- * code must be executed at least once per each half period of the 32-bit
- * counter to properly update the state bit in memory. This is usually not a
- * problem in practice, but if it is then a kernel timer could be scheduled
- * to manage for this code to be executed often enough.
+ * The restrictions for the algorithm to work properly are:
+ *
+ * 1) this code must be called at least once per each half period of the
+ *    32-bit counter;
+ *
+ * 2) this code must not be preempted for a duration longer than the
+ *    32-bit counter half period minus the longest period between two
+ *    calls to this code.
+ *
+ * Those requirements ensure proper update to the state bit in memory.
+ * This is usually not a problem in practice, but if it is then a kernel
+ * timer should be scheduled to manage for this code to be executed often
+ * enough.
  *
  * Note that the top bit (bit 63) in the returned value should be considered
  * as garbage.  It is not cleared here because callers are likely to use a
@@ -68,9 +77,10 @@ union cnt32_to_63 {
  */
 #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(); \
 	__x.lo = (cnt_lo); \
 	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
 		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \