Hi,
Felipe Ferreri Tonello <eu@xxxxxxxxxxxxxxxxx> writes:
> [ text/plain ]
> Hi Balbi,
>
> On 09/03/16 07:20, Felipe Balbi wrote:
>>
>> 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.
>
> Yes, but then what should I do? Because if I patch on Linus' tree, then
> the patch won't apply to your tree.
it should apply to my "fixes" branch where fixes for current -rc should
go :-) Note that v4.5-rc7 doesn't have f_midi_do_transmit()
--
balbi
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