[PATCH] sched: prevent compiler from optimising sched_avg_update loop

[Will Deacon <will.deacon@arm.com>]

GCC 4.4.1 on ARM has been observed to replace the while loop
in sched_avg_update with a call to uldivmod, resulting in the
following build failure at link-time:

kernel/built-in.o: In function `sched_avg_update':
/linux-2.6/kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
/linux-2.6/kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
make: *** [.tmp_vmlinux1] Error 1

This patch [taken against 2.6.34-rc2] introduces a fake data hazard to
the loop body to prevent the compiler optimising the loop away.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 kernel/sched.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 9ab3cd7..0846815 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1240,6 +1240,12 @@ static void sched_avg_update(struct rq *rq)
 	s64 period = sched_avg_period();
 
 	while ((s64)(rq->clock - rq->age_stamp) > period) {
+		/*
+		 * Inline assembly required to prevent the compiler
+		 * optimising this loop into a divmod call.
+		 * See __iter_div_u64_rem() for another example of this.
+		 */
+		asm("" : "+rm" (rq->age_stamp));
 		rq->age_stamp += period;
 		rq->rt_avg /= 2;
 	}
-- 
1.6.3.3