linux-kernel - Re: Chipidea USB device goes infinite loop due to interrupt while hw_ep

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Date:   Mon, 9 Aug 2021 09:27:47 +0800
From:   Peter Chen <peter.chen@...nel.org>
To:     Jeaho Hwang <jhhwang@...t.co.kr>
Cc:     linux-usb@...r.kernel.org,
        변무광(Byeon Moo Kwang)/자동화연)Automation Platform연구팀 
        <mkbyeon@...lectric.co.kr>, Linux team <team-linux@...t.co.kr>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Chipidea USB device goes infinite loop due to interrupt while
 hw_ep_prime

On 21-08-04 11:03:44, Jeaho Hwang wrote:
> Hi. linux-usb and linux-rt experts.
> >
> > On 21-08-02 17:35:01, Jeaho Hwang wrote:
> > > Hi.
> > >
> > > We found an infinite loop inside the function hw_ep_set_halt
> > > (drivers/usb/chipidea/udc.c) if a cablle is repeatedly
> > > connnected/disconnected while ping through RNDIS with chipidea USB device.
> > >
> > > Using ftrace tracing, we found that hw_ep_set_halt is called due to error
> > > return of hw_ep_prime(drivers/usb/chipidea/udc.c:202) which is called from
> > > isr_tr_complete_handler -> isr_setup_status_phase -> _ep_queue.
> > >
> > > The comment of function hw_ep_prime says (execute without interruption) but
> > > timer interrupt is occurred while hw_ep_prime is executing. We believe that
> > > the interrupt causes an error return of hw_ep_prime. We tried to protect
> > > hw_ep_prime from irqs and then no case of the infinite loop is occurred.
> > >
> > > I want ask if it is appropriate way that turning off irq inside (threaded)
> > > irq handlers. And should we explicitly turn off irqs before calling
> > > hw_ep_prime?
> > >
> >
> > Jeaho, do you use RT-Linux or standard Linux? The function hw_ep_prime is
> > only called at udc_irq which is registered as top-half irq handlers.
> > Why the timer interrupt is occurred when hw_ep_prime is executing?
> 
> We use preempt_RT so timer interrupt could be occurred. Now I found
> out that forced threaded irq handler disables local irq on standard
> linux so It is a linux-rt issue. Then should I make patch which
> disables local irqs during hw_ep_prime for RT kernel and suggest it to
> linux-rt maintainers?
> 
> Thanks for a kind answer Peter.
> 

No, that will lead to deadlock since the normal request queue API function
ep_queue disables irq which also calls into hw_ep_prime.

For RT kernel, you may try to disable local irq at isr_setup_status_phase.

-- 

Thanks,
Peter Chen