[PATCH v3] timecounters: Fix timehand generation read/write

[Konstantin Belousov]

On Tue, Jun 09, 2015 at 01:07:54PM +0200, Sebastian Huber wrote:
> You still need the compiler memory barriers in the UP case (at least 
> with GCC 4.9 on PowerPC), e.g. something like this:

Fair.

diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index 9dca0e8..01c61bd 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -189,6 +190,33 @@ tc_delta(struct timehands *th)
 	    tc->tc_counter_mask);
 }
 
+static u_int
+tc_getgen(struct timehands *th)
+{
+
+#ifdef SMP
+	return (atomic_load_acq_int(&th->th_generation));
+#else
+	u_int gen;
+
+	gen = th->th_generation;
+	__compiler_membar();
+	return (gen);
+#endif
+}
+
+static void
+tc_setgen(struct timehands *th, u_int newgen)
+{
+
+#ifdef SMP
+	atomic_store_rel_int(&th->th_generation, newgen);
+#else
+	__compiler_membar();
+	th->th_generation = newgen;
+#endif
+}
+
 /*
  * Functions for reading the time.  We have to loop until we are sure that
  * the timehands that we operated on was not updated under our feet.  See