[PATCH] sched: fix scheduler_disable() with core scheduling

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

In core-scheduling mode, Xen might crash when entering ACPI S5 state.
This happens in sched_slave() during is_idle_unit(next) check because
next->vcpu_list is stale and points to an already freed memory.

This situation happens shortly after scheduler_disable() is called if
some CPU is still inside sched_slave() softirq. Current logic simply
returns prev->next_task from sched_wait_rendezvous_in() which causes
the described crash because next_task->vcpu_list has become invalid.

Fix the crash by returning NULL from sched_wait_rendezvous_in() in
the case when scheduler_disable() has been called.

Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
---
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Dario Faggioli <dfaggioli@xxxxxxxx>
CC: George Dunlap <george.dunlap@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/sched/core.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 626861a3fe..d4a6489929 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -2484,19 +2484,15 @@ static struct sched_unit 
*sched_wait_rendezvous_in(struct sched_unit *prev,
 
         *lock = pcpu_schedule_lock_irq(cpu);
 
-        if ( unlikely(!scheduler_active) )
-        {
-            ASSERT(is_idle_unit(prev));
-            atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
-            prev->rendezvous_in_cnt = 0;
-        }
-
         /*
          * Check for scheduling resource switched. This happens when we are
          * moved away from our cpupool and cpus are subject of the idle
          * scheduler now.
+         *
+         * This is also a bail out case when scheduler_disable() has been
+         * called.
          */
-        if ( unlikely(sr != get_sched_res(cpu)) )
+        if ( unlikely(sr != get_sched_res(cpu) || !scheduler_active) )
         {
             ASSERT(is_idle_unit(prev));
             atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
-- 
2.17.1