rcu: Exit RCU extended QS on user preemption

When exceptions or irq are about to resume userspace, if
the task needs to be rescheduled, the arch low level code
calls schedule() directly.

If we call it, it is because we have the TIF_RESCHED flag:

- It can be set after random local calls to set_need_resched()
(RCU, drm, ...)

- A wake up happened and the CPU needs preemption. This can
  happen in several ways:

    * Remotely: the remote waking CPU has set TIF_RESCHED and send the
      wakee an IPI to schedule the new task.
    * Remotely enqueued: the remote waking CPU sends an IPI to the target
      and the wake up is made by the target.
    * Locally: waking CPU == wakee CPU and the wakeup is done locally.
      set_need_resched() is called without IPI.

In the case of local and remotely enqueued wake ups, the tick can
be restarted when we enqueue the new task and RCU can exit the
extended quiescent state at the same time. Then by the time we reach
irq exit path and we call schedule, we are not in RCU user mode.

But if we call schedule() only because something called set_need_resched(),
RCU may still be in user mode when we reach schedule.

Also if a wake up is done remotely, the CPU might see the TIF_RESCHED
flag and call schedule while the IPI has not yet happen to restart the
tick and exit RCU user mode.

We need to manually protect against these corner cases.

Create a new API schedule_user() that calls schedule() inside
rcu_user_exit()-rcu_user_enter() in order to protect it. Archs
will need to rely on it now to implement user preemption safely.

Signed-off-by: Frederic Weisbecker <[email protected]>
Cc: Alessio Igor Bogani <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Geoff Levand <[email protected]>
Cc: Gilad Ben Yossef <[email protected]>
Cc: Hakan Akkan <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Kevin Hilman <[email protected]>
Cc: Max Krasnyansky <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Sven-Thorsten Dietrich <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>

1 files changed, 15 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4adcd237c545..3c4dec0594d6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3469,6 +3469,21 @@ asmlinkage void __sched schedule(void)
 }
 EXPORT_SYMBOL(schedule);
 
+#ifdef CONFIG_RCU_USER_QS
+asmlinkage void __sched schedule_user(void)
+{
+	/*
+	 * If we come here after a random call to set_need_resched(),
+	 * 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.
+	 */
+	rcu_user_exit();
+	schedule();
+	rcu_user_enter();
+}
+#endif
+
 /**
  * schedule_preempt_disabled - called with preemption disabled
  *