[PATCH 1/1] release_task: kill unnecessary rcu_read_lock() around dec_rlimit_ucounts()

[Oleg Nesterov]

rcu_read_lock() was added to shut RCU-lockdep up when this code used
__task_cred()->rcu_dereference(), but after the commit 21d1c5e386bc
("Reimplement RLIMIT_NPROC on top of ucounts") it is no longer needed:
task_ucounts()->task_cred_xxx() takes rcu_read_lock() itself.

NOTE: task_ucounts() returns the pointer to another rcu-protected data,
struct ucounts. So it should either be used when task->real_cred and thus
task->real_cred->ucounts is stable (release_task, copy_process, copy_creds),
or it should be called under rcu_read_lock(). In both cases it is pointless
to take rcu_read_lock() to read the cred->ucounts pointer.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/exit.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 9f74e8f1c431..f041f0c05ebb 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -251,10 +251,8 @@ void release_task(struct task_struct *p)
 	memset(&post, 0, sizeof(post));
 
 	/* don't need to get the RCU readlock here - the process is dead and
-	 * can't be modifying its own credentials. But shut RCU-lockdep up */
-	rcu_read_lock();
+	 * can't be modifying its own credentials. */
 	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
-	rcu_read_unlock();
 
 	pidfs_exit(p);
 	cgroup_release(p);
-- 
2.25.1.362.g51ebf55