linux-kernel - Re: [PATCH 1/2] usb: gadget: r8a66597: Fix two possible sleep-in-atomic-context bugs in init

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Message-ID: <87a7rdlup1.fsf@belgarion.home>
Date:   Fri, 29 Jun 2018 13:04:58 +0200
From:   Robert Jarzmik <robert.jarzmik@...e.fr>
To:     Felipe Balbi <balbi@...nel.org>
Cc:     Jia-Ju Bai <baijiaju1990@...il.com>, gregkh@...uxfoundation.org,
        nicolas.ferre@...rochip.com, keescook@...omium.org,
        allen.lkml@...il.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] usb: gadget: r8a66597: Fix two possible sleep-in-atomic-context bugs in init_controller()

Felipe Balbi <balbi@...nel.org> writes:

> Hi,
>
> Robert Jarzmik <robert.jarzmik@...e.fr> writes:
>>>>> The driver may sleep with holding a spinlock.
>>>>> The function call paths (from bottom to top) in Linux-4.16.7 are:
>>>>>
>>>>> [FUNC] msleep
>>>>> drivers/usb/gadget/udc/r8a66597-udc.c, 839: 
>>>>> 		msleep in init_controller
>>>>> drivers/usb/gadget/udc/r8a66597-udc.c, 96: 
>>>>> 		init_controller in r8a66597_usb_disconnect
>>>>> drivers/usb/gadget/udc/r8a66597-udc.c, 93: 
>>>>> 		spin_lock in r8a66597_usb_disconnect
>>>>
>>>> That should not happen...
>>>>
>>>> If think the issue you have is that your usb_connect() and usb_disconnect() are
>>>> called from interrupt context. I think the proper fix, as what is done in most
>>>> udc phys, is to schedule a workqueue, see drivers/usb/phy/phy-gpio-vbus-usb.c,
>>>> gpio_vbus_data.vbus.
>>>
>>> argh, no. No workqueues needed here. Sorry
>> Technically why ?
>
> well, strictly technically there's nothing wrong. But it opens a can of
> worms. We've seen time and time again drivers growing into
> unmaintainable mess because of workqueues being fired in several places.
I see.

>>
>> And as bonus question, why is it better to have mdelay() calls in the driver ?
>
> As a bugfix, it's the smallest fix possible, right? Ideally, we wouldn't
> need either of them. Perhaps there's a bit which can be polled instead?
Ideally yes. Do you remember if a "threaded interrupt" might use msleep() ? I
seem to remember that they can, so won't that be another alternative ?

Cheers.

-- 
Robert