tasks, sched/core: With a grace period after finish_task_switch(), remove unnecessary code

Remove work arounds that were written before there was a grace period
after tasks left the runqueue in finish_task_switch().

In particular now that there tasks exiting the runqueue exprience
a RCU grace period none of the work performed by task_rcu_dereference()
excpet the rcu_dereference() is necessary so replace task_rcu_dereference()
with rcu_dereference().

Remove the code in rcuwait_wait_event() that checks to ensure the current
task has not exited.  It is no longer necessary as it is guaranteed
that any running task will experience a RCU grace period after it
leaves the run queueue.

Remove the comment in rcuwait_wake_up() as it is no longer relevant.

Ref: 8f95c90ceb54 ("sched/wait, RCU: Introduce rcuwait machinery")
Ref: 150593bf8693 ("sched/api: Introduce task_rcu_dereference() and try_get_task_struct()")
Signed-off-by: Eric W. Biederman <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Russell King - ARM Linux admin <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

2 files changed, 4 insertions, 17 deletions

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
index 563290fc194f..75c97e4bbc57 100644
--- a/include/linux/rcuwait.h
+++ b/include/linux/rcuwait.h
@@ -6,16 +6,11 @@
 
 /*
  * rcuwait provides a way of blocking and waking up a single
- * task in an rcu-safe manner; where it is forbidden to use
- * after exit_notify(). task_struct is not properly rcu protected,
- * unless dealing with rcu-aware lists, ie: find_task_by_*().
+ * task in an rcu-safe manner.
  *
- * Alternatively we have task_rcu_dereference(), but the return
- * semantics have different implications which would break the
- * wakeup side. The only time @task is non-nil is when a user is
- * blocked (or checking if it needs to) on a condition, and reset
- * as soon as we know that the condition has succeeded and are
- * awoken.
+ * The only time @task is non-nil is when a user is blocked (or
+ * checking if it needs to) on a condition, and reset as soon as we
+ * know that the condition has succeeded and are awoken.
  */
 struct rcuwait {
 	struct task_struct __rcu *task;
@@ -37,13 +32,6 @@ extern void rcuwait_wake_up(struct rcuwait *w);
  */
 #define rcuwait_wait_event(w, condition)				\
 ({									\
-	/*								\
-	 * Complain if we are called after do_exit()/exit_notify(),     \
-	 * as we cannot rely on the rcu critical region for the		\
-	 * wakeup side.							\
-	 */                                                             \
-	WARN_ON(current->exit_state);                                   \
-									\
 	rcu_assign_pointer((w)->task, current);				\
 	for (;;) {							\
 		/*							\
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 153a683646ac..4b1c3b664f51 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -119,7 +119,6 @@ static inline void put_task_struct(struct task_struct *t)
 		__put_task_struct(t);
 }
 
-struct task_struct *task_rcu_dereference(struct task_struct **ptask);
 void put_task_struct_rcu_user(struct task_struct *task);
 
 #ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT