exit: wait: drop tasklist_lock before psig->c* accounting

wait_task_zombie() no longer needs tasklist_lock to accumulate the
psig->c* counters, we can drop it right after cmpxchg(exit_state).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1005,6 +1005,11 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 		EXIT_TRACE : EXIT_DEAD;
 	if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) != EXIT_ZOMBIE)
 		return 0;
+	/*
+	 * We own this thread, nobody else can reap it.
+	 */
+	read_unlock(&tasklist_lock);
+	sched_annotate_sleep();
 
 	/*
 	 * Check thread_group_leader() to exclude the traced sub-threads.
@@ -1064,13 +1069,6 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 		spin_unlock_irq(&current->sighand->siglock);
 	}
 
-	/*
-	 * Now we are sure this task is interesting, and no other
-	 * thread can reap it because we its state == DEAD/TRACE.
-	 */
-	read_unlock(&tasklist_lock);
-	sched_annotate_sleep();
-
 	retval = wo->wo_rusage
 		? getrusage(p, RUSAGE_BOTH, wo->wo_rusage) : 0;
 	status = (p->signal->flags & SIGNAL_GROUP_EXIT)