tasks: Add a count of task RCU users

mainline inclusion
from mainline-5.4-rc1
commit 3fbd7ee2
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I3UKOW
CVE: NA

-------------------------------------------------

Add a count of the number of RCU users (currently 1) of the task
struct so that we can later add the scheduler case and get rid of the
very subtle task_rcu_dereference(), and just use rcu_dereference().

As suggested by Oleg have the count overlap rcu_head so that no
additional space in task_struct is required.
Inspired-by: NLinus Torvalds <torvalds@linux-foundation.org>
Inspired-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/87woebdplt.fsf_-_@x220.int.ebiederm.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NLi Hua <hucool.lihua@huawei.com>
Signed-off-by: NZheng Zucheng <zhengzucheng@huawei.com>

 Conflicts:
         kernel/fork.c
Reviewed-by: NCheng Jian <cj.chengjian@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

include/linux/sched.h

@@ -1087,7 +1087,10 @@ struct task_struct {
 
 	struct tlbflush_unmap_batch	tlb_ubc;
 
-	struct rcu_head			rcu;
+	union {
+		refcount_t		rcu_users;
+		struct rcu_head		rcu;
+	};
 
 	/* Cache last used pipe for splice(): */
 	struct pipe_inode_info		*splice_pipe;

include/linux/sched/task.h

@@ -106,6 +106,7 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
 }
 
 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
 extern int arch_task_struct_size __read_mostly;

kernel/exit.c

@@ -172,6 +172,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
 	put_task_struct(tsk);
 }
 
+void put_task_struct_rcu_user(struct task_struct *task)
+{
+	if (refcount_dec_and_test(&task->rcu_users))
+		call_rcu(&task->rcu, delayed_put_task_struct);
+}
 
 void release_task(struct task_struct *p)
 {
@@ -212,7 +217,7 @@ void release_task(struct task_struct *p)
 
 	write_unlock_irq(&tasklist_lock);
 	release_thread(p);
-	call_rcu(&p->rcu, delayed_put_task_struct);
+	put_task_struct_rcu_user(p);
 
 	p = leader;
 	if (unlikely(zap_leader))

kernel/fork.c

@@ -896,10 +896,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	tsk->stack_canary = get_random_canary();
 #endif
 
-	/*
-	 * One for us, one for whoever does the "release_task()" (usually
-	 * parent)
-	 */
+	/* One for the user space visible state that goes away when reaped. */
+	refcount_set(&tsk->rcu_users, 1);
+	/* One for the rcu users, and one for the scheduler */
 	atomic_set(&tsk->usage, 2);
 #ifdef CONFIG_BLK_DEV_IO_TRACE
 	tsk->btrace_seq = 0;