[Devel] [PATCH RHEL7 COMMIT] ve/sched: Dereference resched_next on next task's stack

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-327.36.1.vz7.20.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-327.36.1.vz7.20.12
------>
commit 53fc9a189617835a17b831512f1f93515c4719aa
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Thu Dec 8 17:45:56 2016 +0400

    ve/sched: Dereference resched_next on next task's stack
    
    When we dereference it before context switch, the read value
    just caches in prev task's stack. Later, the prev task may
    wake up in any cpu, so the value became useless.
    
    https://jira.sw.ru/browse/PSBM-56984
    
    Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
    
    khorenko@: this fixes our priority inversion defense in kernel.
---
 kernel/sched/core.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ccc826a..384373c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3443,9 +3443,6 @@ need_resched:
 	clear_tsk_need_resched(prev);
 	rq->skip_clock_update = 0;
 
-	resched_next = rq->resched_next;
-	rq->resched_next = 0;
-
 	if (likely(prev != next)) {
 		rq->nr_switches++;
 		rq->curr = next;
@@ -3465,8 +3462,11 @@ need_resched:
 
 	post_schedule(rq);
 
-	if (resched_next)
+	resched_next = READ_ONCE(rq->resched_next);
+	if (resched_next) {
 		set_tsk_need_resched(current);
+		rq->resched_next = 0;
+	}
 
 	sched_preempt_enable_no_resched();
 	if (!resched_next && need_resched())