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

[Felipe Balbi <balbi@kernel.org>]

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Hi,

"Felipe F. Tonello" <eu@felipetonello.com> writes:
> [ text/plain ]
> Since f_midi_transmit is called by both ALSA and USB sub-systems, it can
> potentially cause a race condition between both calls because f_midi_transmit
> is not reentrant nor thread-safe. This is due to an implementation detail that
> the transmit function looks for the next available usb request from the fifo
> and only enqueues it if there is data to send, otherwise just re-uses it. So,
> if both ALSA and USB frameworks calls this function at the same time,
> 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.
>
> Cc: <stable@vger.kernel.org> # v4.4+
> Fixes: e1e3d7ec5da3 ("usb: gadget: f_midi: pre-allocate IN requests")
> Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
> Tested-by: Felipe F. Tonello <eu@felipetonello.com>

adding one's own Tested-by is a bit redundant. I'll remove it while
applying ;-)

-- 
balbi

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