Re: [PATCH_TAKE_2] now < last_tick problem

Ian Wienand
2003-10-13T02:11:52Z

[-- Attachment #1: Type: text/plain, Size: 616 bytes --]

On Fri, Oct 10, 2003 at 09:42:35AM -0700, David Mosberger wrote:
> I think the source of the probem is that we consider the value
> returned by get_offset() to be valid EVEN when read_seqretry() returns
> 1. Because of that, we'll end up updating last_nsec_offset with a
> potentialy bad value.

Well, to my eyes the use of the xtime_lock in do_gettimeofday() looks
OK, but I guess what you are saying is that the message is moot --
xtime_lock protects everything itc_get_offset() needs, and
do_gettimeofday() has a read lock on xtime_lock and so reads the
offset again if something was updated underneath it.

-i

[-- Attachment #2: time.c.nomsg.diff --]
[-- Type: text/plain, Size: 1166 bytes --]

===== arch/ia64/kernel/time.c 1.35 vs edited =====
--- 1.35/arch/ia64/kernel/time.c Wed Oct 8 12:53:38 2003
+++ edited/arch/ia64/kernel/time.c Mon Oct 13 11:54:05 2003
@@ -65,8 +65,11 @@
}

/*
- * Return the number of nano-seconds that elapsed since the last update to jiffy. The
- * xtime_lock must be at least read-locked when calling this routine.
+ * Return the number of nano-seconds that elapsed since the last
+ * update to jiffy. It is quite possible that the timer interrupt
+ * will interrupt this and result in a race for any of jiffies,
+ * wall_jiffies or itm_next. Thus, the xtime_lock must be at least
+ * read-locked when calling this routine.
*/
unsigned long
itc_get_offset (void)
@@ -77,11 +80,6 @@
last_tick = (cpu_data(TIME_KEEPER_ID)->itm_next
- (lost + 1)*cpu_data(TIME_KEEPER_ID)->itm_delta);

- if (unlikely((long) (now - last_tick) < 0)) {
- printk(KERN_ERR "CPU %d: now < last_tick (now=0x%lx,last_tick=0x%lx)!\n",
- smp_processor_id(), now, last_tick);
- return last_nsec_offset;
- }
elapsed_cycles = now - last_tick;
return (elapsed_cycles*local_cpu_data->nsec_per_cyc) >> IA64_NSEC_PER_CYC_SHIFT;
}