Re: [uml-devel] actually useful backtrace from a CPU-chewing hang

[Jeff Dike <jdike@addtoit.com>]

On Sat, Apr 26, 2008 at 07:31:44PM +0100, Nix wrote:
> The cause of this is almost certainly time-skewing on the *host* via
> adjtimex(). I stopped ntpd and there were no problems for half a day: I
> restarted it, and as soon as ntpd had synched and begun slewing the time
> (within a second of slewing beginning, probably less than that), *wham*:

Can you try the patch below?  Time should definitely not go backwards
- it's a bug if it happens.  If you want more a more definite
indication about whether this is having an effect, stick a printk in
that test and see if it shows up.

     	      	  Jeff

-- 
Work email - jdike at linux dot intel dot com

Index: linux-2.6.22/arch/um/os-Linux/time.c
===================================================================
--- linux-2.6.22.orig/arch/um/os-Linux/time.c	2008-04-24 13:21:28.000000000 -0400
+++ linux-2.6.22/arch/um/os-Linux/time.c	2008-04-28 12:41:37.000000000 -0400
@@ -107,6 +107,10 @@ static void deliver_alarm(void)
 	unsigned long long this_tick = os_nsecs();
 	int one_tick = UM_NSEC_PER_SEC / UM_HZ;
 
+	/* Protection against the host's time going backwards */
+	if ((last_tick != 0) && (this_tick < last_tick))
+		this_tick = last_tick;
+
 	if (last_tick == 0)
 		last_tick = this_tick - one_tick;
 

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