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authorSalman Qazi <sqazi@google.com>2012-03-09 16:41:01 -0800
committerWilly Tarreau <w@1wt.eu>2012-10-07 23:37:38 +0200
commitab81b6abd9a1cdac0d6e84515cc808f735fc78b7 (patch)
tree694b87cdf2d0f88942a9eda5ad24c4b5866963a5 /include
parent0d1e8b8281ae7b43be54ec3409d299c99d7c975d (diff)
sched/x86: Fix overflow in cyc2ns_offset
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 <sqazi@google.com> Acked-by: John Stultz <john.stultz@linaro.org> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Paul Turner <pjt@google.com> Cc: john stultz <johnstul@us.ibm.com> Link: http://lkml.kernel.org/r/20120310004027.19291.88460.stgit@dungbeetle.mtv.corp.google.com Signed-off-by: Ingo Molnar <mingo@elte.hu> Cc: Mike Galbraith <efault@gmx.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Willy Tarreau <w@1wt.eu>
Diffstat (limited to 'include')
-rw-r--r--include/linux/kernel.h13
1 files changed, 13 insertions, 0 deletions
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 9acb92d8fb06..3526cd4a0066 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -55,6 +55,19 @@ extern const char linux_proc_banner[];
} \
)
+/*
+ * 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; })