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

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Date:	Fri, 04 Mar 2016 09:20:10 +0200
From:	Felipe Balbi <balbi@...nel.org>
To:	"Felipe F. 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,

"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 ?

-- 
balbi

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