linux-kernel - Re: [Intel-wired-lan] [PATCH iwl-next v3] igb: Retrieve Tx timestamp directly from interrupt for i210

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Message-ID: <20260205145104.iWinkXHv@linutronix.de>
Date: Thu, 5 Feb 2026 15:51:04 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: "Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>,
	Kurt Kanzenbach <kurt@...utronix.de>,
	"Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
	"Kitszel, Przemyslaw" <przemyslaw.kitszel@...el.com>,
	Paul Menzel <pmenzel@...gen.mpg.de>,
	"Gomes, Vinicius" <vinicius.gomes@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Richard Cochran <richardcochran@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Eric Dumazet <edumazet@...gle.com>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"Keller, Jacob E" <jacob.e.keller@...el.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v3] igb: Retrieve Tx timestamp
 directly from interrupt for i210

On 2026-02-05 11:56:44 [+0000], Vadim Fedorenko wrote:
> On 05/02/2026 10:37, Loktionov, Aleksandr wrote:
> > spin_lock_irqsave(&wq_head->lock, flags);  <- RT mutex can sleep
> 
> Hmm... that actually means we have some drivers broken for RT kernels if
> they are processing TX timestamps within a single irq vector:
> - hisilicon/hns3
> - intel/i40e (and ice probably)
> - marvell/mvpp2
> 
> For igb/igc/i40e it's still OK to process TX timestamps directly in
> MSI-X configuration, as ring processing has separate vector, right?

The statement made above is not accurate. Each and every driver does
request_irq() and here on PREEMPT_RT you can freely acquire spinlock_t.

But !RT looks problematic…

__skb_tstamp_tx() invokes skb_may_tx_timestamp() which should exit early
most of the time due to the passed bool (which is true) or
sysctl_tstamp_allow_data which is true. However, should both be false
then it tries to
	read_lock_bh(&sk->sk_callback_lock);

where lockdep will complain because this lock is now acquired with
disabled interrupts.

The function will attempt do free the fresh/ cloned skb in error case
via kfree_skb(). Since it is fresh skb, sk_buff::destructor is NULL and
the warning in skb_release_head_state() won't trigger.

So the only thing that bothers me is the read_lock_bh() in
skb_may_tx_timestamp() which deadlocks if the socket is write-locked on
the same CPU.

> But in general skb_tstamp_tx should be moved to BH processing (NAPI poll
> callback).

Sebastian