From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 3 Dec 2014 23:37:08 +0000 (-0800)
Subject: context_tracking: Restore previous state in schedule_user
X-Git-Tag: firefly_0821_release~176^2~2793
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=7cc78f8fa02c2485104b86434acbc1538a3bd807;p=firefly-linux-kernel-4.4.55.git

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>
---

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