Re: [PATCH] wifi: brcmfmac: avoid sleeping tx locks in netpoll context

[Arend van Spriel <arend.vanspriel@broadcom.com>]

On Wed, 30 Sep 2026 20:54:17 +0200, Karl Mehltretter wrote:
> With the default fcmode=0, netpoll calls ndo_start_xmit() with hard
> interrupts disabled and reaches brcmf_sdio_bus_txdata() directly. The
> function takes txq_lock with spin_lock_bh(), and the queue helper takes
> the embedded sk_buff_head lock. These locks may sleep on PREEMPT_RT.
> On non-RT, spin_unlock_bh() can run pending networking softirqs before
> netpoll releases the transmit lock, causing a recursive transmit
> deadlock.
> 
> Use spin_trylock() for IRQ-disabled calls and enqueue with the unlocked
> skb helper while holding txq_lock. On PREEMPT_RT, reject hard IRQ and NMI
> callers, where rt-spinlocks cannot be acquired. If the lock is busy or
> the queue is full, return through the existing drop path. Do not evict an
> older packet from this context. Suppress the queue-full printk because
> netconsole can recursively enter this path.
> 
> The flow-control callback takes another spinlock, so defer it when an
> IRQ-disabled enqueue reaches TXHI. The data worker rechecks the bus state
> and queue length under txq_lock before stopping the queue. Existing TXLOW
> handling wakes the queue after it drains.
> 
> This fixes the direct SDIO transmit path used by fcmode=0. Modes 1 and 2
> take the FWS lock first and need a separate change.
> 
> Fixes: ac3d9dd034e5 ("netpoll: make ndo_poll_controller() optional")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

[...]

> @@ -2796,8 +2797,53 @@ static bool brcmf_sdio_prec_enq(struct pktq *q, struct sk_buff *pkt, int prec)
>  	return p != NULL;
>  }
>  
> +/*
> + * The caller holds txq_lock with hard IRQs disabled. Avoid the skb queue
> + * lock, which may sleep on PREEMPT_RT.
> + */
> +static bool brcmf_sdio_prec_enq_irqoff(struct pktq *q, struct sk_buff *pkt,
> +				       int prec)
> +{
> +	struct sk_buff_head *list = &q->q[prec].skblist;
> +
> +	if (pktq_pfull(q, prec) || pktq_full(q))
> +		return false;
> +
> +	__skb_queue_tail(list, pkt);
> +	q->len++;
> +	if (q->hi_prec < prec)
> +		q->hi_prec = prec;
> +
> +	return true;
> +}

Looking at brcmf_sdio_prec_enq(), it performs essentially the same check on
its fast path (!pktq_pfull(q, prec) && !pktq_full(q)) before calling
brcmu_pktq_penq(), and only falls back to packet eviction when the queue is
full.

Since bus->txq_lock is always held by the caller, using the unlocked
__skb_queue_tail() is safe in both contexts (the embedded skblist spinlock
in brcmu_pktq_penq() has always been redundant in sdio.c).

Is there an opportunity to reuse/refactor brcmf_sdio_prec_enq() rather than
adding a separate brcmf_sdio_prec_enq_irqoff() helper? For example, passing
a can_evict parameter:

static bool brcmf_sdio_prec_enq(struct pktq *q, struct sk_buff *pkt, int prec,
				bool can_evict)
{
	struct sk_buff_head *list = &q->q[prec].skblist;
	struct sk_buff *p;
	int eprec = -1;

	/* Fast case, precedence queue is not full and we are also not
	 * exceeding total queue length
	 */
	if (!pktq_pfull(q, prec) && !pktq_full(q))
		goto enq;

	if (!can_evict)
		return false;

	/* Determine precedence from which to evict packet, if any */
	if (pktq_pfull(q, prec)) {
		eprec = prec;
	} else if (pktq_full(q)) {
		p = brcmu_pktq_peek_tail(q, &eprec);
		if (eprec > prec)
			return false;
	}

	/* Evict if needed */
	if (eprec >= 0) {
		/* Detect queueing to unconfigured precedence */
		if (eprec == prec)
			return false;	/* refuse newer (incoming) packet */
		/* Evict packet according to discard policy */
		p = brcmu_pktq_pdeq_tail(q, eprec);
		if (p == NULL)
			brcmf_err("brcmu_pktq_pdeq_tail() failed\n");
		brcmu_pkt_buf_free_skb(p);
	}

enq:
	__skb_queue_tail(list, pkt);
	q->len++;
	if (q->hi_prec < prec)
		q->hi_prec = prec;

	return true;
}

Then in brcmf_sdio_bus_txdata():

	if (!brcmf_sdio_prec_enq(&bus->txq, pkt, prec, !irq_off)) {
		skb_pull(pkt, bus->tx_hdrlen);
		if (!irq_off)
			brcmf_err("out of bus->txq !!!\n");
		ret = -ENOSR;
	} else {
		ret = 0;
	}

That keeps the queuing logic in one place, eliminates the duplicate helper,
and simplifies the caller.

Regards,
Arend