Re: [PATCH v3 mptcp-net 0/5] mptcp: fix fallback-related races

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo,

On 09/07/2025 17:12, Paolo Abeni wrote:
> This series contains 3 fixes somewhat related to various races we have
> while handling fallback and 2 small follow-up likely more suited for
> net-next.
> 
> The root cause of the issues addressed here is that the check for
> "we can fallback to tcp now" and the related action are not atomic. That
> also applies to fallback due to MP_FAIL - where the window race is even
> wider.
> 
> Address the issue introducing an additional spinlock to bundle together
> all the relevant events, as per patch 1 and 2.
> 
> Note that mptcp_disconnect() unconditionally
> clears the fallback status (zeroing msk->flags) but don't tuch the
> `allows_infinite_fallback` flag. Such issue is addressed in patch 3.
> 
> Patch 4 cleans up a bit the fallback code, introducing specific MIB for
> each FB reason, and patch 5 drops the, hopefully now redundant
> pr_fallback().
> ---
> v2 -> v3:
>  - mptcp_do_fallback  -> mptcp_try_fallback
>  - refactored patch 3/5
>  - changed mibs names, increment fail only when the protocol mandate it
>  - fix W=1 warn in patch 5/5

Thank you for the new version!

It looks good to me, I just a few small questions, not really blocking
the series, and I can eventually do the small modifications when
applying the patches if you prefer.

> 
> Paolo Abeni (5):
>   mptcp: make fallback action and fallback decision atomic
>   mptcp: plug races between subflow fail and subflow creation
>   mptcp: reset fallback status gracefully at disconnect() time
>   mptcp: track fallbacks accurately via mibs
>   mptcp: remove pr_fallback()
> 
>  net/mptcp/ctrl.c     |  4 +-
>  net/mptcp/mib.c      |  5 +++
>  net/mptcp/mib.h      |  7 ++++
>  net/mptcp/options.c  |  6 ++-
>  net/mptcp/pm.c       |  8 +++-
>  net/mptcp/protocol.c | 97 ++++++++++++++++++++++++++++++++++----------
>  net/mptcp/protocol.h | 35 ++++++++--------
>  net/mptcp/subflow.c  | 40 ++++++++++--------
>  8 files changed, 140 insertions(+), 62 deletions(-)
> 

Cheers,
Matt
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