[POWERPC] Fix gettimeofday inaccuracies

There are two problems in the powerpc gettimeofday code which can
cause incorrect results to be returned.

The first is that there is a race between do_gettimeofday and the
timer interrupt:

1. do_gettimeofday does get_tb()

2. decrementer exception on boot cpu which runs timer_recalc_offset,
   which also samples the timebase and updates the do_gtod structure
   with a greater timebase value.

3. do_gettimeofday calls __do_gettimeofday, which leads to the
   negative result from tb_val - temp_varp->tb_orig_stamp.

The second is caused by taking the boot cpu offline, which can cause
the value of tb_last_jiffy to be increased past the currently
available timebase, causing the same underflow as above.

[paulus@samba.org - define and use data_barrier() instead of mb().]

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

2 files changed, 26 insertions, 8 deletions

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 774c0a3c5019..18e59e43d2b3 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -417,7 +417,7 @@ static __inline__ void timer_check_rtc(void)
 /*
  * This version of gettimeofday has microsecond resolution.
  */
-static inline void __do_gettimeofday(struct timeval *tv, u64 tb_val)
+static inline void __do_gettimeofday(struct timeval *tv)
 {
         unsigned long sec, usec;
         u64 tb_ticks, xsec;
@@ -431,7 +431,12 @@ static inline void __do_gettimeofday(struct timeval *tv, u64 tb_val)
          * without a divide (and in fact, without a multiply)
          */
         temp_varp = do_gtod.varp;
-        tb_ticks = tb_val - temp_varp->tb_orig_stamp;
+
+        /* Sampling the time base must be done after loading
+         * do_gtod.varp in order to avoid racing with update_gtod.
+         */
+        data_barrier(temp_varp);
+        tb_ticks = get_tb() - temp_varp->tb_orig_stamp;
         temp_tb_to_xs = temp_varp->tb_to_xs;
         temp_stamp_xsec = temp_varp->stamp_xsec;
         xsec = temp_stamp_xsec + mulhdu(tb_ticks, temp_tb_to_xs);
@@ -464,7 +469,7 @@ void do_gettimeofday(struct timeval *tv)
                 tv->tv_usec = usec;
                 return;
         }
-        __do_gettimeofday(tv, get_tb());
+        __do_gettimeofday(tv);
 }
 
 EXPORT_SYMBOL(do_gettimeofday);
@@ -650,6 +655,7 @@ void timer_interrupt(struct pt_regs * regs)
         int next_dec;
         int cpu = smp_processor_id();
         unsigned long ticks;
+        u64 tb_next_jiffy;
 
 #ifdef CONFIG_PPC32
         if (atomic_read(&ppc_n_lost_interrupts) != 0)
@@ -691,11 +697,14 @@ void timer_interrupt(struct pt_regs * regs)
                         continue;
 
                 write_seqlock(&xtime_lock);
-                tb_last_jiffy += tb_ticks_per_jiffy;
-                tb_last_stamp = per_cpu(last_jiffy, cpu);
-                do_timer(regs);
-                timer_recalc_offset(tb_last_jiffy);
-                timer_check_rtc();
+                tb_next_jiffy = tb_last_jiffy + tb_ticks_per_jiffy;
+                if (per_cpu(last_jiffy, cpu) >= tb_next_jiffy) {
+                        tb_last_jiffy = tb_next_jiffy;
+                        tb_last_stamp = per_cpu(last_jiffy, cpu);
+                        do_timer(regs);
+                        timer_recalc_offset(tb_last_jiffy);
+                        timer_check_rtc();
+                }
                 write_sequnlock(&xtime_lock);
         }
         
diff --git a/include/asm-powerpc/system.h b/include/asm-powerpc/system.h
index 7307aa775671..4c9f5229e833 100644
--- a/include/asm-powerpc/system.h
+++ b/include/asm-powerpc/system.h
@@ -53,6 +53,15 @@
 #define smp_read_barrier_depends()      do { } while(0)
 #endif /* CONFIG_SMP */
 
+/*
+ * This is a barrier which prevents following instructions from being
+ * started until the value of the argument x is known.  For example, if
+ * x is a variable loaded from memory, this prevents following
+ * instructions from being executed until the load has been performed.
+ */
+#define data_barrier(x) \
+        asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");
+
 struct task_struct;
 struct pt_regs;