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Date: Tue, 6 Jan 2004 18:11:57 +0100 (CET)
From: Tim Schmielau <tim@physik3.uni-rostock.de>
To: Linus Torvalds <torvalds@osdl.org>
cc: Andrew Morton <akpm@osdl.org>,
       James Bottomley <James.Bottomley@SteelEye.com>, johnstul@us.ibm.com,
       lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fix get_jiffies_64 to work on voyager
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On Tue, 6 Jan 2004, Linus Torvalds wrote:

> Augh. If you cast it to "unsigned long long" anyway, why not just use the 
> right value? It's "jiffies_64".

Lessons in straight thinking...

We then need to make jiffies_64 volatile, since we violate the rule to 
never read it.

Tim 


--- linux-2.6.1-rc1/arch/i386/kernel/timers/timer_tsc.c	2003-12-31 14:21:19.000000000 +0100
+++ linux-2.6.1-rc1-j64/arch/i386/kernel/timers/timer_tsc.c	2004-01-06 18:05:24.000000000 +0100
@@ -140,7 +140,8 @@
 #ifndef CONFIG_NUMA
 	if (!use_tsc)
 #endif
-		return (unsigned long long)get_jiffies_64() * (1000000000 / HZ);
+		/* no locking but a rare wrong value is not a big deal */
+		return (unsigned long long)jiffies_64 * (1000000000 / HZ);
 
 	/* Read the Time Stamp Counter */
 	rdtscll(this_offset);

--- linux-2.6.1-rc1/include/linux/jiffies.h	2003-12-31 14:22:05.000000000 +0100
+++ linux-2.6.1-rc1-j64/include/linux/jiffies.h	2004-01-06 17:56:15.000000000 +0100
@@ -9,11 +9,11 @@
 #include <asm/param.h>			/* for HZ */
 
 /*
- * The 64-bit value is not volatile - you MUST NOT read it
- * without sampling the sequence number in xtime_lock.
+ * NEVER EVER read jiffies_64 without sampling the sequence number
+ * in xtime_lock.
  * get_jiffies_64() will do this for you as appropriate.
  */
-extern u64 jiffies_64;
+extern u64 volatile jiffies_64;
 extern unsigned long volatile jiffies;
 
 #if (BITS_PER_LONG < 64)
@@ -21,7 +21,7 @@
 #else
 static inline u64 get_jiffies_64(void)
 {
-	return (u64)jiffies;
+	return jiffies_64;
 }
 #endif