[PATCH] sched: Do not re-read h_load_next during hierarchical load calculation

[Mel Gorman <mgorman@techsingularity.net>]

A NULL pointer dereference bug was reported on a distribution kernel but
the same issue should be present on mainline kernel. It occured on s390
but should not be arch-specific.  A partial oops looks like

[775277.408564] Unable to handle kernel pointer dereference in virtual kernel address space
...
[775277.408759] Call Trace:
[775277.408763] ([<0002c11c56899c61>] 0x2c11c56899c61)
[775277.408766]  [<0000000000177bb4>] try_to_wake_up+0xfc/0x450
[775277.408773]  [<000003ff81ede872>] vhost_poll_wakeup+0x3a/0x50 [vhost]
[775277.408777]  [<0000000000194ae4>] __wake_up_common+0xbc/0x178
[775277.408779]  [<0000000000194f86>] __wake_up_common_lock+0x9e/0x160
[775277.408780]  [<00000000001950de>] __wake_up_sync_key+0x4e/0x60
[775277.408785]  [<00000000005d911e>] sock_def_readable+0x5e/0x98

The bug hits any time between 1 hour to 3 days. The dereference occurs
in update_cfs_rq_h_load when accumulating h_load. The problem is that
cfq_rq->h_load_next is not protected by any locking and can be updated
by parallel calls to task_h_load. Depending on the compiler, code may be
generated that re-reads cfq_rq->h_load_next after the check for NULL and
then oops when reading se->avg.load_avg. The dissassembly showed that it
was possible to reread h_load_next after the check for NULL.

While this does not appear to be an issue for later compilers, it's still
an accident if the correct code is generated. Full locking in this path
would have high overhead so this patch uses READ_ONCE to read h_load_next
only once and check for NULL before dereferencing. It was confirmed that
there were no further oops after 10 days of testing.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 310d0637fe4b..34aeb40e69d2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7726,7 +7726,7 @@ static void update_cfs_rq_h_load(struct cfs_rq *cfs_rq)
 		cfs_rq->last_h_load_update = now;
 	}
 
-	while ((se = cfs_rq->h_load_next) != NULL) {
+	while ((se = READ_ONCE(cfs_rq->h_load_next)) != NULL) {
 		load = cfs_rq->h_load;
 		load = div64_ul(load * se->avg.load_avg,
 			cfs_rq_load_avg(cfs_rq) + 1);