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Felipe Ferreri Tonello <e...@felipetonello.com> writes: > [ text/plain ] > Hi Balbi, > > On March 4, 2016 7:20:10 AM GMT+00:00, Felipe Balbi <ba...@kernel.org> wrote: >> >>Hi, >> >>"Felipe F. Tonello" <e...@felipetonello.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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