March 20, 2025 at 08:06, "Cong Wang" xiyou.wangcong@gmail.com wrote:
On Wed, Mar 19, 2025 at 11:36:13PM +0000, Jiayuan Chen wrote:
2025/3/20 07:02, "Cong Wang" xiyou.wangcong@gmail.com wrote:
On Mon, Mar 17, 2025 at 05:22:54PM +0800, Jiayuan Chen wrote:
The sk->sk_socket is not locked or referenced, and during the call to
Hm? We should have a reference in socket map, whether directly or
indirectly, right? When we add a socket to a socket map, we do call
sock_map_psock_get_checked() to obtain a reference.
Yes, but we remove psock from sockmap when sock_map_close() was called
'''
sock_map_close
lock_sock(sk);
rcu_read_lock();
psock = sk_psock(sk);
// here we remove psock and the reference of psock become 0
sock_map_remove_links(sk, psock)
sk_psock_drop() also calls cancel_delayed_work_sync(&psock->work),
althrough in yet another work. Is this also a contribution to this bug?
Maybe it's related. Calling cancel_delayed_work_sync() in sk_psock_drop() is too late for our scenario.
To be more precise, the core goal of this patch is to prevent sock_map_close() from executing until the backlog work completes. This is because sock_map_close() resides in the close(fd) path, once it finishes, subsequent steps will release the sk_socket. Therefore, performing cancellation in sk_psock_drop() is too late.
Upon reviewing historical commits, I found that the backlog work originally held lock_sk, which naturally synchronized with lock_sk in sock_map_close. However, when the backlog work later removed lock_sk, an alternative synchronization mechanism(just hold psock reference like this patch) became necessary.
psock = sk_psock_get(sk);
if (unlikely(!psock))
goto no_psock; <=== jmp to no_psock
rcu_read_unlock();
release_sock(sk);
cancel_delayed_work_sync(&psock->work); <== no chance to run cancel
'''
I have to say sock_map_close() becomes harder and harder to understand
now. And I am feeling we may have more bugs since we have two flying
work's here: psock->rwork and psock->work.
Thanks.
Yes, this patch prevent sock_map_close() from executing until the backlog work completes. This likely makes the cancel_delayed_work in sk_psock_destroy redundant.
The code has undergone too many iterations. While sk_psock_destroy certainly contains redundant operations, we should retain it for now. There may be hidden dependencies we haven't fully untangled.
Thanks.