[PATCH v2] usb: cdns3: Fix deadlock when using NCM gadget

[Ralph Siemsen]

The cdns3 driver has the same NCM deadlock as fixed in cdnsp by commit
58f2fcb3a845 ("usb: cdnsp: Fix deadlock issue during using NCM gadget").

Under PREEMPT_RT the deadlock can be readily triggered by heavy network
traffic, for example using "iperf --bidir" over NCM ethernet link.

The deadlock occurs because the threaded interrupt handler gets
preempted by a softirq, but both are protected by the same spinlock.
Prevent deadlock by disabling softirq during threaded irq handler.

cc: stable@xxxxxxxxxxxxxxx
Fixes: 7733f6c32e36 ("usb: cdns3: Add Cadence USB3 DRD Driver")
Signed-off-by: Ralph Siemsen <ralph.siemsen@xxxxxxxxxx>
---
v2 changes:
- move the fix up the call stack, as per discussion at
https://lore.kernel.org/linux-rt-devel/20250226082931.-XRIDa6D@xxxxxxxxxxxxx/
---
 drivers/usb/cdns3/cdns3-gadget.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/usb/cdns3/cdns3-gadget.c b/drivers/usb/cdns3/cdns3-gadget.c
index fd1beb10bba72..19101ff1cf1bd 100644
--- a/drivers/usb/cdns3/cdns3-gadget.c
+++ b/drivers/usb/cdns3/cdns3-gadget.c
@@ -1963,6 +1963,7 @@ static irqreturn_t cdns3_device_thread_irq_handler(int irq, void *data)
 	unsigned int bit;
 	unsigned long reg;
 
+	local_bh_disable();
 	spin_lock_irqsave(&priv_dev->lock, flags);
 
 	reg = readl(&priv_dev->regs->usb_ists);
@@ -2004,6 +2005,7 @@ static irqreturn_t cdns3_device_thread_irq_handler(int irq, void *data)
 irqend:
 	writel(~0, &priv_dev->regs->ep_ien);
 	spin_unlock_irqrestore(&priv_dev->lock, flags);
+	local_bh_enable();
 
 	return ret;
 }

---
base-commit: 4701f33a10702d5fc577c32434eb62adde0a1ae1
change-id: 20250312-rfs-cdns3-deadlock-697df5cad3ce

Best regards,
-- 
Ralph Siemsen <ralph.siemsen@xxxxxxxxxx>