release_task: kill the no longer needed get/put_pid(thread_pid)

After the commit 7903f907a2 ("pid: perform free_pid() calls outside
of tasklist_lock") __unhash_process() -> detach_pid() no longer calls
free_pid(), proc_flush_pid() can just use p->thread_pid without the
now pointless get_pid() + put_pid().

Signed-off-by: Oleg Nesterov <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Mateusz Guzik <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

kernel/exit.c

@@ -258,7 +258,8 @@ repeat:
 	pidfs_exit(p);
 	cgroup_release(p);
 
-	thread_pid = get_pid(p->thread_pid);
+	/* Retrieve @thread_pid before __unhash_process() may set it to NULL. */
+	thread_pid = task_pid(p);
 
 	write_lock_irq(&tasklist_lock);
 	ptrace_release_task(p);
@@ -287,8 +288,8 @@ repeat:
 	}
 
 	write_unlock_irq(&tasklist_lock);
+	/* @thread_pid can't go away until free_pids() below */
 	proc_flush_pid(thread_pid);
-	put_pid(thread_pid);
 	add_device_randomness(&p->se.sum_exec_runtime,
 			      sizeof(p->se.sum_exec_runtime));
 	free_pids(post.pids);