Messages in this thread | | | Date | Mon, 21 Aug 2023 10:55:58 -0700 | Subject | Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of next_thread() | From | Kui-Feng Lee <> |
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On 8/21/23 08:09, Oleg Nesterov wrote: > 1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we > can safely iterate the task->thread_group list. Even if this task exits > right after get_pid_task() (or goto retry) and pid_alive() returns 0 > > Kill the unnecessary pid_alive() check.
This function will return next_task holding a refcount, and release the refcount until the next time calling the same function. Meanwhile, the returned task A may be killed, and its next task B may be killed after A as well, before calling this function again. However, even task B is destroyed (free), A's next is still pointing to task B. When this function is called again for the same iterator, it doesn't promise that B is still there.
Does that make sense to you?
> > 2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)" > check. > > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > --- > kernel/bpf/task_iter.c | 7 ------- > 1 file changed, 7 deletions(-) > > diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c > index c4ab9d6cdbe9..4d1125108014 100644 > --- a/kernel/bpf/task_iter.c > +++ b/kernel/bpf/task_iter.c > @@ -75,15 +75,8 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm > return NULL; > > retry: > - if (!pid_alive(task)) { > - put_task_struct(task); > - return NULL; > - } > - > next_task = next_thread(task); > put_task_struct(task); > - if (!next_task) > - return NULL; > > saved_tid = *tid; > *tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
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