[PATCH v2 3/4] task: With a grace period after finish_task_switch, remove unnecessary code

[Eric W. Biederman]

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.

Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
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" <ebiederm@xxxxxxxxxxxx>
---
 include/linux/rcuwait.h    | 20 +++---------
 include/linux/sched/task.h |  1 -
 kernel/exit.c              | 67 --------------------------------------
 kernel/sched/fair.c        |  2 +-
 kernel/sched/membarrier.c  |  4 +--
 5 files changed, 7 insertions(+), 87 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 4c44c37236b2..8bd51af44bf8 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -115,7 +115,6 @@ 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
diff --git a/kernel/exit.c b/kernel/exit.c
index 2e259286f4e7..f943773622fc 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -234,69 +234,6 @@ void release_task(struct task_struct *p)
 		goto repeat;
 }
 
-/*
- * Note that if this function returns a valid task_struct pointer (!NULL)
- * task->usage must remain >0 for the duration of the RCU critical section.
- */
-struct task_struct *task_rcu_dereference(struct task_struct **ptask)
-{
-	struct sighand_struct *sighand;
-	struct task_struct *task;
-
-	/*
-	 * We need to verify that release_task() was not called and thus
-	 * delayed_put_task_struct() can't run and drop the last reference
-	 * before rcu_read_unlock(). We check task->sighand != NULL,
-	 * but we can read the already freed and reused memory.
-	 */
-retry:
-	task = rcu_dereference(*ptask);
-	if (!task)
-		return NULL;
-
-	probe_kernel_address(&task->sighand, sighand);
-
-	/*
-	 * Pairs with atomic_dec_and_test() in put_task_struct(). If this task
-	 * was already freed we can not miss the preceding update of this
-	 * pointer.
-	 */
-	smp_rmb();
-	if (unlikely(task != READ_ONCE(*ptask)))
-		goto retry;
-
-	/*
-	 * We've re-checked that "task == *ptask", now we have two different
-	 * cases:
-	 *
-	 * 1. This is actually the same task/task_struct. In this case
-	 *    sighand != NULL tells us it is still alive.
-	 *
-	 * 2. This is another task which got the same memory for task_struct.
-	 *    We can't know this of course, and we can not trust
-	 *    sighand != NULL.
-	 *
-	 *    In this case we actually return a random value, but this is
-	 *    correct.
-	 *
-	 *    If we return NULL - we can pretend that we actually noticed that
-	 *    *ptask was updated when the previous task has exited. Or pretend
-	 *    that probe_slab_address(&sighand) reads NULL.
-	 *
-	 *    If we return the new task (because sighand is not NULL for any
-	 *    reason) - this is fine too. This (new) task can't go away before
-	 *    another gp pass.
-	 *
-	 *    And note: We could even eliminate the false positive if re-read
-	 *    task->sighand once again to avoid the falsely NULL. But this case
-	 *    is very unlikely so we don't care.
-	 */
-	if (!sighand)
-		return NULL;
-
-	return task;
-}
-
 void rcuwait_wake_up(struct rcuwait *w)
 {
 	struct task_struct *task;
@@ -316,10 +253,6 @@ void rcuwait_wake_up(struct rcuwait *w)
 	 */
 	smp_mb(); /* (B) */
 
-	/*
-	 * Avoid using task_rcu_dereference() magic as long as we are careful,
-	 * see comment in rcuwait_wait_event() regarding ->exit_state.
-	 */
 	task = rcu_dereference(w->task);
 	if (task)
 		wake_up_process(task);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bc9cfeaac8bd..215c640e2a6b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1644,7 +1644,7 @@ static void task_numa_compare(struct task_numa_env *env,
 		return;
 
 	rcu_read_lock();
-	cur = task_rcu_dereference(&dst_rq->curr);
+	cur = rcu_dereference(dst_rq->curr);
 	if (cur && ((cur->flags & PF_EXITING) || is_idle_task(cur)))
 		cur = NULL;
 
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index aa8d75804108..b14250a11608 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -71,7 +71,7 @@ static int membarrier_global_expedited(void)
 			continue;
 
 		rcu_read_lock();
-		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
+		p = rcu_dereference(cpu_rq(cpu)->curr);
 		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
 				   MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
 			if (!fallback)
@@ -150,7 +150,7 @@ static int membarrier_private_expedited(int flags)
 		if (cpu == raw_smp_processor_id())
 			continue;
 		rcu_read_lock();
-		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
+		p = rcu_dereference(cpu_rq(cpu)->curr);
 		if (p && p->mm == current->mm) {
 			if (!fallback)
 				__cpumask_set_cpu(cpu, tmpmask);
-- 
2.21.0.dirty