linux-kernel - Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

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Date:   Sat, 12 Nov 2016 15:35:02 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, linux-usb@...r.kernel.org,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

Hi Peter,

On 11/11/2016 08:28 PM, Peter Zijlstra wrote:
> On Fri, Nov 11, 2016 at 12:33:29PM +0800, Lu Baolu wrote:
>
>> Things become complicated when it comes to USB debug port.
>> But it's still addressable.
>>
>> At this time, we can do it like this.
>>
>> write()
>> {
>> 	if (in_nmi() && raw_spin_is_locked(&lock))
>> 		return;
>>
>> 	raw_spinlock_irqsave(&lock, flags);
>> 	....
>>
> Please use raw_spin_trlock_irqsave() instead, spin_is_locked() is fairly
> icky.

Sure.

>
> Also, there's a bunch of exception contexts that do not set in_nmi().
> That is in_nmi() is really only set for #NM. #MC and #DB and
> others do not set this.

That's worth another fix patch. Let me look into it later.

>
>> This will filter some messages from NMI handler in case that
>> another thread is holding the spinlock. I have no idea about
>> how much chance could a debug user faces this. But it might
>> further be fixed with below enhancement.
>>
>> write()
>> {
>> 	if (in_nmi() && raw_spin_is_locked(&lock)) {
>> 		produce_a_pending_item_in_ring();
>> 		return;
>> 	}
>>
>> 	raw_spinlock_irqsave(&lock, flags);
>>
>> 	while (!pending_item_ring_is_empty)
>> 		consume_a_pending_item_in_ring();
>>
>> 	....
>>
>>
>> We can design the pending item ring in a producer-consumer
>> model. It's easy to avoid race between the producer and
>> consumer.
> Problem is that the consumer might never happen, those are the fun most
> bugs.
>
> Not being able to deal with random nested exception context really
> reduces the utility of this thing.
>
> Again, a UART rules. Make a virtual UART in hardware, that'd be totally
> awesome. This thing, I'm not convinced its worth having.

This is the initial work. It helps at least in cases where people need
to dump kernel messages but lacking of a console. It's also a cheap
way, people don't need to buy any third-party devices.

With more and more people trying and enhancing it, it will become
more robust and helpful.

Best regards,
Lu Baolu