Re: [PATCH 2/5] usb: gadget: f_midi: added spinlock on transmit function

[Felipe Ferreri Tonello <eu@felipetonello.com>]

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Hi Balbi,

On 07/03/16 07:32, Felipe Balbi wrote:
> 
> Hi,
> 
> (please break your lines at 80-characters, have a look at
> Documentation/email-clients.txt if needed)
> 
> Felipe Ferreri Tonello <eu@felipetonello.com> writes:
>> [ text/plain ]
>> Hi Balbi, 
>>
>> On March 4, 2016 7:20:10 AM GMT+00:00, Felipe Balbi <balbi@kernel.org> wrote:
>>>
>>> Hi,
>>>
>>> "Felipe F. Tonello" <eu@felipetonello.com> writes:
>>>> [ text/plain ]
>>>> Since f_midi_transmit is called by both ALSA and USB frameworks, it
>>> can
>>>> potentially cause a race condition between both calls. This is bad
>>> because the
>>>> way f_midi_transmit is implemented can't handle concurrent calls.
>>> This is due
>>>> to the fact that the usb request fifo looks for the next element and
>>> only if
>>>> it has data to process it enqueues the request, otherwise re-uses it.
>>> If both
>>>> (ALSA and USB) frameworks calls this function at the same time, the
>>>> kfifo_seek() will return the same usb_request, which will cause a
>>> race
>>>> condition.
>>>>
>>>> To solve this problem a syncronization mechanism is necessary. In
>>> this case it
>>>> is used a spinlock since f_midi_transmit is also called by
>>> usb_request->complete
>>>> callback in interrupt context.
>>>>
>>>> On benchmarks realized by me, spinlocks were more efficient then
>>> scheduling
>>>> the f_midi_transmit tasklet in process context and using a mutex to
>>>> synchronize. Also it performs better then previous implementation
>>> that
>>>> allocated a usb_request for every new transmit made.
>>>
>>> behaves better in what way ? Also, previous implementation would not
>>> suffer from this concurrency problem, right ?
>>
>> The spin lock is faster than allocating usb requests all the time,
>> even if the udc uses da for it.
> 
> did you measure ? Is the extra speed really necessary ? How did you
> benchmark this ?

Yes I did measure and it was not that significant. This is not about
speed. There was a bug in that approach that I already explained on that
patch, which was approved and applied BTW.

Any way, this spinlock should've been there since that patch but I
couldn't really trigger this problem without a stress test.

So, this patch fixes a bug in the current implementation.

Felipe

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