[PATCH] usb: mtu3: don't run gadget completion callbacks with interrupts disabled

[Ryan Brue <ryanbrue.dev@gmail.com>]

mtu3_irq() is a threaded handler, so it runs in process context, but it
takes mtu->lock with spin_lock_irqsave(). mtu3_req_complete() then drops
the lock with a plain spin_unlock() around
usb_gadget_giveback_request(), which leaves hardware interrupts disabled
across the gadget function's completion callback.

That breaks any callback that reaches netif_rx(). Since the threaded
handler is in neither hardirq nor softirq context, netif_rx() takes its
local_bh_disable()/local_bh_enable() path, and enabling softirqs with
interrupts disabled is not allowed:

  WARNING: kernel/softirq.c:430 at __local_bh_enable_ip+0x17c/0x188
    __local_bh_enable_ip
    netif_rx
    rx_complete [u_ether]
    usb_gadget_giveback_request
    mtu3_req_complete
    mtu3_qmu_isr
    mtu3_irq
    irq_thread_fn

With CONFIG_TRACE_IRQFLAGS, __local_bh_enable_ip() ends with an
unconditional local_irq_enable(), so the plain spin_lock() re-taking
mtu->lock afterwards runs with interrupts on and is recorded
SOFTIRQ-ON-W, while mtu3_gadget_queue() takes the same lock from a
softirq (ncm's tx hrtimer), and lockdep follows up with an inconsistent
lock state report naming mtu->lock.

The locking was right when mtu3_irq() was a hardirq handler. It stopped
being right when the handler became threaded without a primary handler;
dwc3 hit the same thing the same way and fixed it in the controller
driver, in commit 84918a89d6ef ("usb: dwc3: gadget: Let the interrupt
handler disable bottom halves.").

mtu->lock is never taken in hardirq context: the only handler is
threaded and has no primary handler, so disabling softirqs is enough.
Take it with spin_lock_bh() in mtu3_irq(). The completion callback then
runs with interrupts enabled and softirqs disabled, which is the context
netif_rx() expects. The other sites keep spin_lock_irqsave(), which is
strictly stronger and still correct; several of them really can be
reached from softirq context.

Both reports need lockdep to be seen, but the context violation is real:
netif_rx() wants either hardirq/softirq context or interrupts enabled,
and mtu3 gives it neither. The fallout on a PROVE_LOCKING kernel is not
cosmetic either - copying a 21 MB file over USB ethernet hung and reset
this board repeatedly before this change. Once interrupts have been
left enabled while the thread holds mtu->lock, the tx hrtimer softirq on
the same CPU spins on it forever, which is the "CPU0: lock(&mtu->lock);
<Interrupt> lock(&mtu->lock);" scenario lockdep names. The same copy is
reliable after it.

Fixes: 13118959cb1a ("usb: mtu3: register mtu3_irq by threaded irq")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Ryan Brue <ryanbrue.dev@gmail.com>
---
 drivers/usb/mtu3/mtu3_core.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/mtu3/mtu3_core.c b/drivers/usb/mtu3/mtu3_core.c
index a40bf5bad2d5..ed0e426645fd 100644
--- a/drivers/usb/mtu3/mtu3_core.c
+++ b/drivers/usb/mtu3/mtu3_core.c
@@ -790,10 +790,9 @@ static irqreturn_t mtu3_u2_common_isr(struct mtu3 *mtu)
 static irqreturn_t mtu3_irq(int irq, void *data)
 {
 	struct mtu3 *mtu = (struct mtu3 *)data;
-	unsigned long flags;
 	u32 level1;
 
-	spin_lock_irqsave(&mtu->lock, flags);
+	spin_lock_bh(&mtu->lock);
 
 	/* U3D_LV1ISR is RU */
 	level1 = mtu3_readl(mtu->mac_base, U3D_LV1ISR);
@@ -814,7 +813,7 @@ static irqreturn_t mtu3_irq(int irq, void *data)
 	if (level1 & QMU_INTR)
 		mtu3_qmu_isr(mtu);
 
-	spin_unlock_irqrestore(&mtu->lock, flags);
+	spin_unlock_bh(&mtu->lock);
 
 	return IRQ_HANDLED;
 }

---
base-commit: df2908090cda368b01ff43709f51890076c56157
change-id: 20260913-usb-mtu3-gadget-completion-callback-fix-d334f7e99a17

Best regards,
--  
Ryan Brue <ryanbrue.dev@gmail.com>