This is a note to let you know that I've just added the patch titled
sched/x86: Fix overflow in cyc2ns_offset
to the 3.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-x86-fix-overflow-in-cyc2ns_offset.patch
and it can be found in the queue-3.0 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 Mon Sep 17 00:00:00 2001
From: Salman Qazi <[email protected]>
Date: Fri, 9 Mar 2012 16:41:01 -0800
Subject: sched/x86: Fix overflow in cyc2ns_offset
From: Salman Qazi <[email protected]>
commit 9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 upstream.
When a machine boots up, the TSC generally gets reset. However,
when kexec is used to boot into a kernel, the TSC value would be
carried over from the previous kernel. The computation of
cycns_offset in set_cyc2ns_scale is prone to an overflow, if the
machine has been up more than 208 days prior to the kexec. The
overflow happens when we multiply *scale, even though there is
enough room to store the final answer.
We fix this issue by decomposing tsc_now into the quotient and
remainder of division by CYC2NS_SCALE_FACTOR and then performing
the multiplication separately on the two components.
Refactor code to share the calculation with the previous
fix in __cycles_2_ns().
Signed-off-by: Salman Qazi <[email protected]>
Acked-by: John Stultz <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: john stultz <[email protected]>
Link:
http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Mike Galbraith <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
arch/x86/include/asm/timer.h | 8 ++------
arch/x86/kernel/tsc.c | 3 ++-
include/linux/kernel.h | 13 +++++++++++++
3 files changed, 17 insertions(+), 7 deletions(-)
--- a/arch/x86/include/asm/timer.h
+++ b/arch/x86/include/asm/timer.h
@@ -57,14 +57,10 @@ DECLARE_PER_CPU(unsigned long long, cyc2
static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
{
- unsigned long long quot;
- unsigned long long rem;
int cpu = smp_processor_id();
unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
- quot = (cyc >> CYC2NS_SCALE_FACTOR);
- rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
- ns += quot * per_cpu(cyc2ns, cpu) +
- ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
+ ns += mult_frac(cyc, per_cpu(cyc2ns, cpu),
+ (1UL << CYC2NS_SCALE_FACTOR));
return ns;
}
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -623,7 +623,8 @@ static void set_cyc2ns_scale(unsigned lo
if (cpu_khz) {
*scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
- *offset = ns_now - (tsc_now * *scale >> CYC2NS_SCALE_FACTOR);
+ *offset = ns_now - mult_frac(tsc_now, *scale,
+ (1UL << CYC2NS_SCALE_FACTOR));
}
sched_clock_idle_wakeup_event(0);
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -77,6 +77,19 @@
} \
)
+/*
+ * Multiplies an integer by a fraction, while avoiding unnecessary
+ * overflow or loss of precision.
+ */
+#define mult_frac(x, numer, denom)( \
+{ \
+ typeof(x) quot = (x) / (denom); \
+ typeof(x) rem = (x) % (denom); \
+ (quot * (numer)) + ((rem * (numer)) / (denom)); \
+} \
+)
+
+
#define _RET_IP_ (unsigned long)__builtin_return_address(0)
#define _THIS_IP_ ({ __label__ __here; __here: (unsigned long)&&__here; })
Patches currently in stable-queue which might be from [email protected] are
queue-3.0/sched-x86-fix-overflow-in-cyc2ns_offset.patch
--
To unsubscribe from this list: send the line "unsubscribe stable" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
