On Sun, Jan 08, 2012 at 11:19:14AM +0100, Peter Zijlstra wrote: > But even then, we limit PROP_MAX_SHIFT to 3*BITS_PER_LONG/4, I don't > think that could ever overflow. do_div() only uses the lower 32 bit value of the 64 bit denominator, which may happen to be 0. The denominator is not really 64 bit, but limited by PROP_MAX_SHIFT = 48 bit, however that upper limit looks not enough. Ilya, would you help try the attached patch instead? Thanks, Fengguang
Subject: Date: Mon Jan 09 11:53:50 CST 2012 Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> --- include/linux/proportions.h | 4 ++++ 1 file changed, 4 insertions(+) --- linux.orig/include/linux/proportions.h 2012-01-09 11:51:19.000000000 +0800 +++ linux/include/linux/proportions.h 2012-01-09 11:53:47.000000000 +0800 @@ -81,7 +81,11 @@ void prop_inc_percpu(struct prop_descrip * Limit the time part in order to ensure there are some bits left for the * cycle counter and fraction multiply. */ +#if BITS_PER_LONG == 32 #define PROP_MAX_SHIFT (3*BITS_PER_LONG/4) +#else +#define PROP_MAX_SHIFT (BITS_PER_LONG/2) +#endif #define PROP_FRAC_SHIFT (BITS_PER_LONG - PROP_MAX_SHIFT - 1) #define PROP_FRAC_BASE (1UL << PROP_FRAC_SHIFT)
