[PATCH] now < last_tick problem

[PATCH] now < last_tick problem

[Ian Wienand]

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Hi,
                                                                                                                                                    
I started hitting this problem when using httperf, upon investigation
it seems to be easily triggered when you make gettimeofday() calls in
very close succession (just doing them in a tight loop easily
replicates the problem).
                                                                                                                                                    
I think the problem is that in itc_get_offset() there is a race
between getting the now value and calculating the last_tick from
itm_next; if we are interrupted between the two itm_next will be set
after now.
                                                                                                                                                    
Suggested patch attached; note the fsyscall implementation does not
appear to have this problem.
                                                                                                                                                    
-i
ianw@gelato.unsw.edu.au
http://www.gelato.unsw.edu.au

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===== 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	Fri Oct 10 09:49:37 2003
@@ -72,10 +72,16 @@
 itc_get_offset (void)
 {
 	unsigned long elapsed_cycles, lost = jiffies - wall_jiffies;
-	unsigned long now = ia64_get_itc(), last_tick;
+	unsigned long now, last_tick;
 
 	last_tick = (cpu_data(TIME_KEEPER_ID)->itm_next
 		     - (lost + 1)*cpu_data(TIME_KEEPER_ID)->itm_delta);
+	
+	/*
+	 * get now after last_tick to avoid race condition where
+	 * itm_next might be updated.
+	 */
+	now = ia64_get_itc();
 
 	if (unlikely((long) (now - last_tick) < 0)) {
 		printk(KERN_ERR "CPU %d: now < last_tick (now=0x%lx,last_tick=0x%lx)!\n",