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

[Breno Leitao]

Hello Matthieu,

On Wed, Oct 30, 2024 at 03:45:02PM +0100, Matthieu Baerts wrote:
> Hi Breno
> 
> 30 Oct 2024 15:02:45 Breno Leitao <leitao@xxxxxxxxxx>:
> 
> > 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@xxxxxxxxxx/
> 
> 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?

Oh, I was testing on Linus' tree when I got the problem, and net was not
merged in Linus' tree yet.

> > 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.

Let me comment there.

> 
> > 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.

Thanks