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 <[email protected]> Cc: Oleg Nesterov <[email protected]> Cc: Frédéric Weisbecker <[email protected]> Acked-by: Paul E. McKenney <[email protected]> Signed-off-by: Andy Lutomirski <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
cbmeeks · Dec 4, 2014 · 7cc78f8 · 7cc78f8
1 parent ebcd241
commit 7cc78f8
Showing 1 changed file with 6 additions and 2 deletions.
diff --git 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