Re: [PATCH net] mptcp: Ensure RCU read lock is held when calling mptcp_sched_find()

[Matthieu Baerts <matttbe@kernel.org>]

Hi Breno

30 Oct 2024 15:02:45 Breno Leitao <leitao@debian.org>:

> The mptcp_sched_find() function must be called with the RCU read lock
> held, as it accesses RCU-protected data structures. This requirement was
> not properly enforced in the mptcp_init_sock() function, leading to a
> RCU list traversal in a non-reader section error when
> CONFIG_PROVE_RCU_LIST is enabled.
>
>     net/mptcp/sched.c:44 RCU-list traversed in non-reader section!!
>
> Fix it by acquiring the RCU read lock before calling the
> mptcp_sched_find() function. This ensures that the function is invoked
> with the necessary RCU protection in place, as it accesses RCU-protected
> data structures.

Thank you for having looked at that, but there is already a fix:

https://lore.kernel.org/netdev/20241021-net-mptcp-sched-lock-v1-1-637759cf061c@kernel.org/

This fix has even been applied in the net tree already:

https://git.kernel.org/netdev/net/c/3deb12c788c3

Did you not get conflicts when rebasing your branch on top of the
latest version?

> Additionally, the patch breaks down the mptcp_init_sched() call into
> smaller parts, with the RCU read lock only covering the specific call to
> mptcp_sched_find(). This helps minimize the critical section, reducing
> the time during which RCU grace periods are blocked.

I agree with Eric (thank you for the review!): this creates other issues.

> The mptcp_sched_list_lock is not held in this case, and it is not clear
> if it is necessary.

It is not needed, the list is not modified, only read with RCU.

Cheers,
Matt