linux-kernel - Re: 2.6.23-rc3-mm1 - vdso and gettimeofday issues with glibc

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Date:	Sun, 9 Sep 2007 09:27:06 +0200
From:	Andi Kleen <ak@...e.de>
To:	Valdis.Kletnieks@...edu
Cc:	Chuck Ebbert <cebbert@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, jakub@...hat.com,
	Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.23-rc3-mm1 - vdso and gettimeofday issues with glibc


> Updating on this issue:   Both myself and another person have reported on
> the RedHat bugzilla that it's a clocksource issue - if you are using the
> hpet clocksource, the time warps, but booting with clocksource=acpi_pm works.
> 
> This ring any bells?

Does this patch fix it? 
-Andi

Add missing mask operation to vdso

vdso vgetns() didn't mask the time source offset calculation, which could
lead to time problems with 32bit HPET. Add the masking.

Thanks to Chuck Ebbert for tracking down.

Signed-off-by: Andi Kleen <ak@...e.de>

Index: linux/arch/x86_64/vdso/vclock_gettime.c
===================================================================
--- linux.orig/arch/x86_64/vdso/vclock_gettime.c
+++ linux/arch/x86_64/vdso/vclock_gettime.c
@@ -34,10 +34,11 @@ static long vdso_fallback_gettime(long c
 
 static inline long vgetns(void)
 {
+	long v;
 	cycles_t (*vread)(void);
 	vread = gtod->clock.vread;
-	return ((vread() - gtod->clock.cycle_last) * gtod->clock.mult) >>
-		gtod->clock.shift;
+	v = (vread() - gtod->clock.cycle_last) & gtod->clock.mask;
+	return (v * gtod->clock.mult) >> gtod->clock.shift;
 }
 
 static noinline int do_realtime(struct timespec *ts)
-
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