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Commit 9993bc63 authored by Salman Qazi's avatar Salman Qazi Committed by Ingo Molnar
Browse files

sched/x86: Fix overflow in cyc2ns_offset 



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: default avatarSalman Qazi <sqazi@google.com>
Acked-by: default avatarJohn Stultz <john.stultz@linaro.org>
Acked-by: default avatarPeter 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: default avatarIngo Molnar <mingo@elte.hu>
parent 47258cf3

arch/x86/include/asm/timer.h

+2 −6
Original line number Diff line number Diff line
@@ -57,14 +57,10 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset); 


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;

}

arch/x86/kernel/tsc.c

+2 −1
Original line number Diff line number Diff line
@@ -620,7 +620,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu) 


	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);

include/linux/kernel.h

+13 −0
Original line number Diff line number Diff line
@@ -85,6 +85,19 @@ 
}							\

)



/*

 * 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; })