context_tracking: Restore preempted context state after preempt_schedule_irq()

From the context tracking POV, preempt_schedule_irq() behaves pretty much
like an exception: It can be called anytime and schedule another task.

But currently it doesn't restore the context tracking state of the preempted
code on preempt_schedule_irq() return.

As a result, if preempt_schedule_irq() is called in the tiny frame between
user_enter() and the actual return to userspace, we resume userspace with
the wrong context tracking state.

Fix this by using exception_enter/exit() which are a perfect fit for this
kind of issue.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Mats Liljegren <mats.liljegren@enea.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f12624a393c..af7a8c84b797 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3082,11 +3082,13 @@ EXPORT_SYMBOL(preempt_schedule);
 asmlinkage void __sched preempt_schedule_irq(void)
 {
         struct thread_info *ti = current_thread_info();
+        enum ctx_state prev_state;
 
         /* Catch callers which need to be fixed */
         BUG_ON(ti->preempt_count || !irqs_disabled());
 
-        user_exit();
+        prev_state = exception_enter();
+
         do {
                 add_preempt_count(PREEMPT_ACTIVE);
                 local_irq_enable();
@@ -3100,6 +3102,8 @@ asmlinkage void __sched preempt_schedule_irq(void)
                  */
                 barrier();
         } while (need_resched());
+
+        exception_exit(prev_state);
 }
 
 #endif /* CONFIG_PREEMPT */