netdev - Re: [PATCH net v3] ptp: prevent possible ABBA deadlock in ptp_clock

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Message-ID: <20250721083011.zesywxhisw435g73@skbuf>
Date: Mon, 21 Jul 2025 11:30:11 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Jeongjun Park <aha310510@...il.com>
Cc: richardcochran@...il.com, andrew+netdev@...n.ch, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	yangbo.lu@....com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzbot+7cfb66a237c4a5fb22ad@...kaller.appspotmail.com
Subject: Re: [PATCH net v3] ptp: prevent possible ABBA deadlock in
 ptp_clock_freerun()

On Sat, Jul 19, 2025 at 09:40:22PM +0900, Jeongjun Park wrote:
> diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c
> index 7febfdcbde8b..b16c66c254ae 100644
> --- a/drivers/ptp/ptp_vclock.c
> +++ b/drivers/ptp/ptp_vclock.c
> @@ -154,6 +154,20 @@ static long ptp_vclock_refresh(struct ptp_clock_info *ptp)
>  	return PTP_VCLOCK_REFRESH_INTERVAL;
>  }
>  
> +#ifdef CONFIG_LOCKDEP
> +static void ptp_vclock_set_subclass(struct ptp_clock *ptp)
> +{
> +	lockdep_set_subclass(&ptp->n_vclocks_mux, PTP_LOCK_VIRTUAL);
> +	lockdep_set_subclass(&ptp->clock.rwsem, PTP_LOCK_VIRTUAL);
> +	lockdep_set_subclass(&ptp->tsevqs_lock, PTP_LOCK_VIRTUAL);
> +	lockdep_set_subclass(&ptp->pincfg_mux, PTP_LOCK_VIRTUAL);

Every other lock except &ptp->clock.rwsem is unrelated, and I wouldn't
touch what is unrelated as part of a bug fix. That, plus I believe this
breaks the data encapsulation of struct posix_clock. At least CC the
"POSIX CLOCKS and TIMERS" maintainers in v4, so that they're aware of
your intentions.

> +}
> +#else
> +static void ptp_vclock_set_subclass(struct ptp_clock *ptp)
> +{
> +}
> +#endif
> +

lockdep_set_subclass() has shim definitions for CONFIG_LOCKDEP=n, you
don't need the #ifdef.