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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-09-30 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.

Is this the only affected driver?
Do you have maybe a backtrace?

> Fixes: ac3d9dd034e5 ("netpoll: make ndo_poll_controller() optional")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Sebastian