Re: [RFC PATCH mptcp-next] mptcp: annotate data-races around sysctl reads

[gang.yan@linux.dev]

August 14, 2026 at 11:27 AM, "Tao Cui" <cui.tao@linux.dev mailto:cui.tao@linux.dev?to=%22Tao%20Cui%22%20%3Ccui.tao%40linux.dev%3E > wrote:

Hi, Tao

Thanks for your patch, but it has been submitted by Matt before in [1].

I think Matt wanted change the PM and sched sysctl from string to atomically,
that may need another patches (READ_ONCE is not enough), right? If no, you can
wait for the other maintainers' reply.

If yes, I still think this patch should keep author as matt, the rest of
others can be yours.

Note: Some patches about sched is only in our export branch, not mainlined,
so it's better to do your work based on it.

[1] https://patchwork.kernel.org/project/mptcp/patch/20260601-mptcp-add-addr6-port-ts-fixes-v2-v1-4-d7c842e80446@kernel.org/

Thanks
Gang

> 
> From: Tao Cui <cuitao@kylinos.cn>
> 
> #626 plans a READ_ONCE-over-sysctls series for the string sysctls; this
> is the numeric side. Asking whether to send it on its own or fold it in.
> 
> The per-netns MPTCP sysctl values (net.mptcp.enabled, add_addr_timeout,
> checksum_enabled, allow_join_initial_addr_port, stale_loss_cnt,
> close_timeout, pm_type) are written from the sysctl handlers and read
> without locking through the ctrl.c accessors.
> 
> Add READ_ONCE() on the readers and WRITE_ONCE() on the pm_type store in
> proc_path_manager(), matching what is already done for other mptcp
> fields (fully_established, local_id, remote_id, sysctl_tcp_wmem[0]).
> 
> The string sysctls (path_manager, scheduler) are not covered: they need
> atomic replacement, see the tracker below.
> 
> KCSAN reproduces the race on net.mptcp.enabled, and the READ_ONCE makes
> it go away:
> 
>  BUG: KCSAN: data-race in mptcp_is_enabled / proc_dou8vec_minmax
> 
>  write to 0xffff8f93c18553e9 of 1 bytes by task 214 on cpu 1:
>  proc_dou8vec_minmax+0x1b1/0x200
>  proc_sys_call_handler+0x268/0x350
>  vfs_write+0x423/0x710
> 
>  read to 0xffff8f93c18553e9 of 1 bytes by task 72 on cpu 0:
>  mptcp_is_enabled+0x50/0x60
>  mptcp_init_sock+0x2a/0x1c0
>  inet_create+0x3f9/0x5a0
> 
>  value changed: 0x00 -> 0x01
> 
> No functional change.
> 
> Link: https://github.com/multipath-tcp/mptcp_net-next/issues/626
> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> ---
>  net/mptcp/ctrl.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
> index 63c5747f0f63..b0ef6aea4eba 100644
> --- a/net/mptcp/ctrl.c
> +++ b/net/mptcp/ctrl.c
> @@ -50,39 +50,39 @@ static struct mptcp_pernet *mptcp_get_pernet(const struct net *net)
> 
>  int mptcp_is_enabled(const struct net *net)
>  {
> - return mptcp_get_pernet(net)->mptcp_enabled;
> + return READ_ONCE(mptcp_get_pernet(net)->mptcp_enabled);
>  }
> 
>  unsigned int mptcp_get_add_addr_timeout(const struct net *net)
>  {
> - return mptcp_get_pernet(net)->add_addr_timeout;
> + return READ_ONCE(mptcp_get_pernet(net)->add_addr_timeout);
>  }
> 
>  int mptcp_is_checksum_enabled(const struct net *net)
>  {
> - return mptcp_get_pernet(net)->checksum_enabled;
> + return READ_ONCE(mptcp_get_pernet(net)->checksum_enabled);
>  }
> 
>  int mptcp_allow_join_id0(const struct net *net)
>  {
> - return mptcp_get_pernet(net)->allow_join_initial_addr_port;
> + return READ_ONCE(mptcp_get_pernet(net)->allow_join_initial_addr_port);
>  }
> 
>  unsigned int mptcp_stale_loss_cnt(const struct net *net)
>  {
> - return mptcp_get_pernet(net)->stale_loss_cnt;
> + return READ_ONCE(mptcp_get_pernet(net)->stale_loss_cnt);
>  }
> 
>  unsigned int mptcp_close_timeout(const struct sock *sk)
>  {
>  if (sock_flag(sk, SOCK_DEAD))
>  return TCP_TIMEWAIT_LEN;
> - return mptcp_get_pernet(sock_net(sk))->close_timeout;
> + return READ_ONCE(mptcp_get_pernet(sock_net(sk))->close_timeout);
>  }
> 
>  int mptcp_get_pm_type(const struct net *net)
>  {
> - return mptcp_get_pernet(net)->pm_type;
> + return READ_ONCE(mptcp_get_pernet(net)->pm_type);
>  }
> 
>  const char *mptcp_get_path_manager(const struct net *net)
> @@ -230,7 +230,7 @@ static int proc_path_manager(const struct ctl_table *ctl, int write,
>  pm_type = MPTCP_PM_TYPE_KERNEL;
>  else if (strncmp(pm_name, "userspace", MPTCP_PM_NAME_MAX) == 0)
>  pm_type = MPTCP_PM_TYPE_USERSPACE;
> - pernet->pm_type = pm_type;
> + WRITE_ONCE(pernet->pm_type, pm_type);
>  }
>  }
> 
> --
> 2.43.0
>