android-mainline/Revert-sched-core-Prevent-race-condition-between-cpuset-and-__sched_setscheduler.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Todd Kjos <tkjos@google.com>
Date: Fri, 27 Mar 2020 10:04:16 -0700
Subject: Revert "sched/core: Prevent race condition between cpuset and
 __sched_setscheduler()"

This reverts commit 710da3c8ea7dfbd327920afd3831d8c82c42789d.

When changing a thread's scheduling priority, binder calls
sched_setscheduler_nocheck() while holding the node lock and
proc inner lock. This was safe until v5.3 when this change was
introduced where cpuset_read_lock() is called in this path
which can sleep.

This patch was introduced to fix a possible accounting error
in sched deadline (potential oversell of CPU bandwidth) due
to a race condition between cpusets and __sched_setscheduler().
This is not an issue for Android.

This should be fixed in the binder driver, but that may take
some time.

[CPNOTE: 21/07/21] Lee: Requested status from the author via the bug
[CPNOTE: 07/09/21] Lee: Required whilst carrying current Binder PI implementation

Bug: 120446518 # OoT
Bug: 151861772
Change-Id: Ica1ef71b3cdcdc509b341ea1b57c41f8ee73794a
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Lee Jones <joneslee@google.com>
---
 include/linux/cpuset.h |  5 -----
 kernel/cgroup/cpuset.c | 11 -----------
 kernel/sched/core.c    | 20 +++-----------------
 3 files changed, 3 insertions(+), 33 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -71,8 +71,6 @@ extern void cpuset_init_smp(void);
 extern void cpuset_force_rebuild(void);
 extern void cpuset_update_active_cpus(void);
 extern void cpuset_wait_for_hotplug(void);
-extern void cpuset_read_lock(void);
-extern void cpuset_read_unlock(void);
 extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
 extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
 extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
@@ -196,9 +194,6 @@ static inline void cpuset_update_active_cpus(void)
 
 static inline void cpuset_wait_for_hotplug(void) { }
 
-static inline void cpuset_read_lock(void) { }
-static inline void cpuset_read_unlock(void) { }
-
 static inline void cpuset_cpus_allowed(struct task_struct *p,
 				       struct cpumask *mask)
 {
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -404,17 +404,6 @@ static struct cpuset top_cpuset = {
  */
 
 DEFINE_STATIC_PERCPU_RWSEM(cpuset_rwsem);
-
-void cpuset_read_lock(void)
-{
-	percpu_down_read(&cpuset_rwsem);
-}
-
-void cpuset_read_unlock(void)
-{
-	percpu_up_read(&cpuset_rwsem);
-}
-
 static DEFINE_SPINLOCK(callback_lock);
 
 static struct workqueue_struct *cpuset_migrate_mm_wq;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7515,9 +7515,6 @@ static int __sched_setscheduler(struct task_struct *p,
 			return retval;
 	}
 
-	if (pi)
-		cpuset_read_lock();
-
 	/*
 	 * Make sure no PI-waiters arrive (or leave) while we are
 	 * changing the priority of the task:
@@ -7592,8 +7589,6 @@ static int __sched_setscheduler(struct task_struct *p,
 	if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
 		policy = oldpolicy = -1;
 		task_rq_unlock(rq, p, &rf);
-		if (pi)
-			cpuset_read_unlock();
 		goto recheck;
 	}
 
@@ -7659,10 +7654,8 @@ static int __sched_setscheduler(struct task_struct *p,
 	head = splice_balance_callbacks(rq);
 	task_rq_unlock(rq, p, &rf);
 
-	if (pi) {
-		cpuset_read_unlock();
+	if (pi)
 		rt_mutex_adjust_pi(p);
-	}
 
 	/* Run balance callbacks after we've adjusted the PI chain: */
 	balance_callbacks(rq, head);
@@ -7672,8 +7665,6 @@ static int __sched_setscheduler(struct task_struct *p,
 
 unlock:
 	task_rq_unlock(rq, p, &rf);
-	if (pi)
-		cpuset_read_unlock();
 	return retval;
 }
 
@@ -7803,14 +7794,9 @@ do_sched_setscheduler(pid_t pid, int policy, struct sched_param __user *param)
 	rcu_read_lock();
 	retval = -ESRCH;
 	p = find_process_by_pid(pid);
-	if (likely(p))
-		get_task_struct(p);
-	rcu_read_unlock();
-
-	if (likely(p)) {
+	if (p != NULL)
 		retval = sched_setscheduler(p, policy, &lparam);
-		put_task_struct(p);
-	}
+	rcu_read_unlock();
 
 	return retval;
 }