linux-kernel - Re: [syzbot] [can?] KCSAN: data-race in can_send / can

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Message-ID: <CAMZ6RqKga=f=Xd33GF1zPwmiearrz3mg+ZiryVbJD_RE5MGjKA@mail.gmail.com>
Date: Mon, 10 Mar 2025 18:55:06 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Oliver Hartkopp <socketcan@...tkopp.net>
Cc: mkl@...gutronix.de, 
	syzbot <syzbot+78ce4489b812515d5e4d@...kaller.appspotmail.com>, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	linux-can@...r.kernel.org
Subject: Re: [syzbot] [can?] KCSAN: data-race in can_send / can_send (5)

On Mon. 10 Mar 2025 at 18:46, Oliver Hartkopp <socketcan@...tkopp.net> wrote:
> On 10.03.25 10:29, Vincent Mailhol wrote:
> > On Mon. 10 Mar 2025 at 03:59, Oliver Hartkopp <socketcan@...tkopp.net> wrote:

(...)

> >> Isn't there some lock-less per-cpu safe statistic handling within netdev
> >> we might pick for our use-case?
> >
> > I see two solutions. Either we use lock_sock(skb->sk) and
> > release_sock(skb->sk) or we can change the types of
> > can_pkg_stats->tx_frames and can_pkg_stats->tx_frames_delta from long
> > to atomic_long_t.
> >
> > The atomic_long_t is the closest solution to a lock-less. But my
> > preference goes to the lock_sock() which looks more natural in this
> > context. And look_sock() is just a spinlock which under the hood is
> > also an atomic, so no big penalty either.
>
> When we get skbs from the netdevice (and not from user space), we do not
> have a valid sk value. It is set to zero.
>
> See:
> https://elixir.bootlin.com/linux/v6.13.6/source/net/can/raw.c#L203
>
> And those skbs can also be forwarded by can-gw using can_send().
>
> Therefore there is no lock_sock() without a valid sk ;-)
>
> When 'atomic_long_t' would also fix this simple statistics handling, we
> should use that.

I see, Thanks for the explanation. Then atomic_long_t seems the best
(and easiest).