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Date: Sat, 2 Sep 2006 12:32:29 -0700
From: Andrew Morton <akpm@osdl.org>
To: tglx@linutronix.de
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
       Frank v Waveren <fvw@var.cx>
Subject: Re: [PATCH] prevent timespec/timeval to ktime_t overflow
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On Sat, 02 Sep 2006 20:41:33 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Fri, 2006-09-01 at 20:13 -0700, Andrew Morton wrote:
> > > Fun, here is a version with a bigabyte blocker.
> > 
> > Your patch triggers waaaaaaay early.
> > 
> > netconsole: remote IP 192.168.2.33
> > netconsole: remote ethernet address 00:0d:56:c6:c6:cc
> > Initializing CPU#0
> > PID hash table entries: 4096 (order: 12, 32768 bytes)
> > time.c: Using 14.318180 MHz WALL HPET GTOD HPET/TSC timer.
> > time.c: Detected 3400.238 MHz processor.
> > ktime_set: -1157140842 : 0
> 
> -------------^   !!!!!!!
> 
> > BUG: warning at include/linux/ktime.h:84/ktime_set()
> > 
> > Call Trace:
> >  <IRQ> [<ffffffff80247021>] hrtimer_run_queues+0x10e/0x211
> 
> This seems to happen inside hrtimer_get_softirq_time().
> wall_to_monotonic is negative.
> 
> Why does the check trigger ? We compare a "long", which contains a
> negative value against some positive constant. 
> 
> ktime_t ktime_set(const long secs, const unsigned long nsecs)
> {
> 	if (unlikely(secs >= KTIME_SEC_MAX)) {
> 
> where KTIME_SEC_MAX is 0x7FFFFFFFFFFFFFFF / 1000 000 000 = 
> 
> 9223372036 == 0x225C17D04,
> 
> which is compared against 
> 
> -1157140842 == 0xFFFFFFFFBB076E96
> 
> This smells like gcc magic. Can you please disassemble the code in
> question ?
> 

ktime_set:
.LFB522:
	.file 1 "kernel/hrtimer.c"
	.loc 1 884 0
.LVL0:
	pushq	%rbp	#
.LCFI0:
	movq	%rsp, %rbp	#,
.LCFI1:
.LBB2:
.LBB3:
.LBB4:
	.file 2 "include/asm/current.h"
	.loc 2 11 0
#APP
	movq %gs:0,%rax	#, t
.LVL1:
#NO_APP
.LBE4:
.LBE3:
.LBE2:
	.loc 1 888 0
	cmpq	$0, 208(%rax)	#, <variable>.mm
	je	.L2	#,
	movabsq	$9223372035, %rax	#, tmp66
.LVL2:
	cmpq	%rax, %rdi	# tmp66, secs
	jbe	.L2	#,
	.loc 1 889 0
	cmpl	$0, no88bigabytes.11819(%rip)	#, no88bigabytes
	jne	.L5	#,
	.loc 1 890 0
	movl	$1, no88bigabytes.11819(%rip)	#, no88bigabytes
	.loc 1 891 0
	movq	%rsi, %rdx	# nsecs, nsecs
	movq	%rdi, %rsi	# secs, secs


-- 
VGER BF report: H 0.000301227
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