context_tracking: Restore previous state in schedule_user

It appears that some SCHEDULE_USER (asm for schedule_user) callers
in arch/x86/kernel/entry_64.S are called from RCU kernel context,
and schedule_user will return in RCU user context.  This causes RCU
warnings and possible failures.

This is intended to be a minimal fix suitable for 3.18.

Reported-and-tested-by: Dave Jones <davej@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 24beb9bb4c3e..89e7283015a6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2874,10 +2874,14 @@ asmlinkage __visible void __sched schedule_user(void)
          * or we have been woken up remotely but the IPI has not yet arrived,
          * we haven't yet exited the RCU idle mode. Do it here manually until
          * we find a better solution.
+         *
+         * NB: There are buggy callers of this function.  Ideally we
+         * should warn if prev_state != IN_USER, but that will trigger
+         * too frequently to make sense yet.
          */
-        user_exit();
+        enum ctx_state prev_state = exception_enter();
         schedule();
-        user_enter();
+        exception_exit(prev_state);
 }
 #endif