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

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Hi,

"Felipe F. Tonello" <eu@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> # v4.4+

this should be v4.5+

$ git describe e1e3d7ec5da3
v4.4-rc5-59-ge1e3d7ec5da3

> @@ -651,17 +653,22 @@ static void f_midi_transmit(struct f_midi *midi)
>  {
>  	struct usb_ep *ep = midi->in_ep;
>  	int ret;
> +	unsigned long flags;
>  
>  	/* We only care about USB requests if IN endpoint is enabled */
>  	if (!ep || !ep->enabled)
>  		goto drop_out;
>  
> +	spin_lock_irqsave(&midi->transmit_lock, flags);
> +
>  	do {
>  		ret = f_midi_do_transmit(midi, ep);

you wrote this commit on top of 'next' right ? I know that because of
this call to f_midi_do_transmit() here which is introduced by commit
aac887442d5e ("usb: gadget: f_midi: move some of f_midi_transmit to
separate func") which is not in Linus' tree yet.

This prevents me from taking this patch during current -rc.

-- 
balbi

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