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

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Date:	Mon, 07 Mar 2016 09:32:29 +0200
From:	Felipe Balbi <balbi@...nel.org>
To:	Felipe Ferreri Tonello <eu@...ipetonello.com>,
	linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Michal Nazarewicz <mina86@...a86.com>,
	Clemens Ladisch <clemens@...isch.de>
Subject: Re: [PATCH 2/5] usb: gadget: f_midi: added spinlock on transmit function


Hi,

(please break your lines at 80-characters, have a look at
Documentation/email-clients.txt if needed)

Felipe Ferreri Tonello <eu@...ipetonello.com> writes:
> [ text/plain ]
> Hi Balbi, 
>
> On March 4, 2016 7:20:10 AM GMT+00:00, Felipe Balbi <balbi@...nel.org> wrote:
>>
>>Hi,
>>
>>"Felipe F. Tonello" <eu@...ipetonello.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 ?

-- 
balbi

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