[PATCH v2] usb: mtu3: disable bottom halves in the threaded interrupt handler

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

mtu3_irq() is a threaded handler, so it runs in process context, and it
takes mtu->lock with spin_lock_irqsave(). mtu3_req_complete() drops the
lock with a plain spin_unlock() around usb_gadget_giveback_request(), so
the gadget function's completion callback runs with interrupts disabled,
as the gadget API promises, but in neither hardirq nor softirq context.

That breaks any callback that reaches netif_rx(). netif_rx() disables
bottom halves itself when it is called outside hardirq and softirq
context, and enabling them again with interrupts disabled is not
allowed:

  WARNING: kernel/softirq.c:453 at __local_bh_enable_ip+0x128/0x134
    __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.").

Do the same: disable bottom halves around the lock in mtu3_irq().
netif_rx() then finds softirqs disabled and leaves them alone, the
completion callbacks keep running with interrupts disabled, and the
softirq it raises runs at local_bh_enable(), once the lock is dropped
and interrupts are on again.

Both reports need lockdep to be seen, but the context violation is real.
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. 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 with this change applied.

Fixes: 13118959cb1a ("usb: mtu3: register mtu3_irq by threaded irq")
Cc: stable@vger.kernel.org
Signed-off-by: Ryan Brue <ryanbrue.dev@gmail.com>
---
v1 took mtu->lock with spin_lock_bh() in mtu3_irq(), which left
interrupts enabled across the gadget completion callbacks. Sashiko
pointed out that struct usb_request's @complete must run with them
disabled, and that u_serial's console relies on it [1]. v2 keeps
spin_lock_irqsave() and disables bottom halves around it, as dwc3 does.

This patch was developed with the help of an LLM.

[1] https://lore.kernel.org/all/sashiko-outbox-193807@kernel.org/
---
Changes in v2:
- Keep interrupts disabled across the completion callbacks: disable
  bottom halves around the existing spin_lock_irqsave() instead of
  taking the lock with spin_lock_bh() (Sashiko)
- New subject; v1 was "usb: mtu3: don't run gadget completion callbacks
  with interrupts disabled"
- Rebased onto v7.4-rc1
- v1 was resent on 2026-09-29 and 2026-11-02:
  https://lore.kernel.org/all/20261102-usb-mtu3-gadget-completion-callback-fix-v1-1-93c3a6dda31f@gmail.com/
- Link to v1: https://patch.msgid.link/20260913-usb-mtu3-gadget-completion-callback-fix-v1-1-e3480004fe23@gmail.com
---
 drivers/usb/mtu3/mtu3_core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/usb/mtu3/mtu3_core.c b/drivers/usb/mtu3/mtu3_core.c
index 990e2e8e6bc2..3127709acc22 100644
--- a/drivers/usb/mtu3/mtu3_core.c
+++ b/drivers/usb/mtu3/mtu3_core.c
@@ -793,6 +793,7 @@ static irqreturn_t mtu3_irq(int irq, void *data)
 	unsigned long flags;
 	u32 level1;
 
+	local_bh_disable();
 	spin_lock_irqsave(&mtu->lock, flags);
 
 	/* U3D_LV1ISR is RU */
@@ -815,6 +816,7 @@ static irqreturn_t mtu3_irq(int irq, void *data)
 		mtu3_qmu_isr(mtu);
 
 	spin_unlock_irqrestore(&mtu->lock, flags);
+	local_bh_enable();
 
 	return IRQ_HANDLED;
 }

---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20260913-usb-mtu3-gadget-completion-callback-fix-d334f7e99a17

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