[PATCH] usb: dwc3: gadget: Let the interrupt handler disable bottom halves.

[PATCH] usb: dwc3: gadget: Let the interrupt handler disable bottom halves.

[Sebastian Andrzej Siewior]

The interrupt service routine registered for the gadget is a primary
handler which mask the interrupt source and a threaded handler which
handles the source of the interrupt. Since the threaded handler is
voluntary threaded, the IRQ-core does not disable bottom halves before
invoke the handler like it does for the forced-threaded handler.

Due to changes in networking it became visible that a network gadget's
completions handler may schedule a softirq which remains unprocessed.
The gadget's completion handler is usually invoked either in hard-IRQ or
soft-IRQ context. In this context it is enough to just raise the softirq
because the softirq itself will be handled once that context is left.
In the case of the voluntary threaded handler, there is nothing that
will process pending softirqs. Which means it remain queued until
another random interrupt (on this CPU) fires and handles it on its exit
path or another thread locks and unlocks a lock with the bh suffix.
Worst case is that the CPU goes idle and the NOHZ complains about
unhandled softirqs.

Disable bottom halves before acquiring the lock (and disabling
interrupts) and enable them after dropping the lock. This ensures that
any pending softirqs will handled right away.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lkml.kernel.org/r/c2a64979-73d1-2c22-e048-c275c9f81558@samsung.com
Fixes: e5f68b4a3e7b0 ("Revert "usb: dwc3: gadget: remove unnecessary _irqsave()"")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/usb/dwc3/gadget.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index 183b90923f51b..a0c883f19a417 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -4160,9 +4160,11 @@ static irqreturn_t dwc3_thread_interrupt(int irq, void *_evt)
 	unsigned long flags;
 	irqreturn_t ret = IRQ_NONE;
 
+	local_bh_disable();
 	spin_lock_irqsave(&dwc->lock, flags);
 	ret = dwc3_process_event_buf(evt);
 	spin_unlock_irqrestore(&dwc->lock, flags);
+	local_bh_enable();
 
 	return ret;
 }
-- 
2.34.1

Re: [PATCH] usb: dwc3: gadget: Let the interrupt handler disable bottom halves.

[Marek Szyprowski]

On 18.02.2022 18:32, Sebastian Andrzej Siewior wrote:
> The interrupt service routine registered for the gadget is a primary
> handler which mask the interrupt source and a threaded handler which
> handles the source of the interrupt. Since the threaded handler is
> voluntary threaded, the IRQ-core does not disable bottom halves before
> invoke the handler like it does for the forced-threaded handler.
>
> Due to changes in networking it became visible that a network gadget's
> completions handler may schedule a softirq which remains unprocessed.
> The gadget's completion handler is usually invoked either in hard-IRQ or
> soft-IRQ context. In this context it is enough to just raise the softirq
> because the softirq itself will be handled once that context is left.
> In the case of the voluntary threaded handler, there is nothing that
> will process pending softirqs. Which means it remain queued until
> another random interrupt (on this CPU) fires and handles it on its exit
> path or another thread locks and unlocks a lock with the bh suffix.
> Worst case is that the CPU goes idle and the NOHZ complains about
> unhandled softirqs.
>
> Disable bottom halves before acquiring the lock (and disabling
> interrupts) and enable them after dropping the lock. This ensures that
> any pending softirqs will handled right away.
>
> Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Link: https://lkml.kernel.org/r/c2a64979-73d1-2c22-e048-c275c9f81558@samsung.com
> Fixes: e5f68b4a3e7b0 ("Revert "usb: dwc3: gadget: remove unnecessary _irqsave()"")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
>   drivers/usb/dwc3/gadget.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 183b90923f51b..a0c883f19a417 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -4160,9 +4160,11 @@ static irqreturn_t dwc3_thread_interrupt(int irq, void *_evt)
>   	unsigned long flags;
>   	irqreturn_t ret = IRQ_NONE;
>   
> +	local_bh_disable();
>   	spin_lock_irqsave(&dwc->lock, flags);
>   	ret = dwc3_process_event_buf(evt);
>   	spin_unlock_irqrestore(&dwc->lock, flags);
> +	local_bh_enable();
>   
>   	return ret;
>   }

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland