sched: Prevent compiler from optimising the sched_avg_update() loop

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':
 kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
 kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
make: *** [.tmp_vmlinux1] Error 1

This patch introduces a fake data hazard to the loop body to
prevent the compiler optimising the loop away.

Signed-off-by: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Russell King <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 6 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 6c9e7c8735bf..a24d6d5d83f6 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1254,6 +1254,12 @@ 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;
 	}