Hi Hans, On Friday 22 November 2013 10:02:49 Hans Verkuil wrote: > On 11/21/2013 08:04 PM, Laurent Pinchart wrote: > > On Thursday 21 November 2013 16:21:59 Hans Verkuil wrote: > >> From: Hans Verkuil <hans.verkuil@xxxxxxxxx> > >> > >> Rather than taking the mmap semaphore at a relatively high-level > >> function, push it down to the place where it is really needed. > >> > >> It was placed in vb2_queue_or_prepare_buf() to prevent racing with other > >> vb2 calls, however, I see no way that any race can happen. > > > > What about the following scenario ? Both QBUF calls are performed on the > > same buffer. > > > > CPU 0 CPU 1 > > ------------------------------------------------------------------------- > > QBUF QBUF > > locks the queue mutex waits for the queue mutex > > vb2_qbuf > > vb2_queue_or_prepare_buf > > __vb2_qbuf > > checks vb->state, calls > > __buf_prepare > > call_qop(q, wait_prepare, q); > > unlocks the queue mutex > > > > locks the queue mutex > > vb2_qbuf > > vb2_queue_or_prepare_buf > > __vb2_qbuf > > checks vb->state, calls > > __buf_prepare > > call_qop(q, wait_prepare, q); > > unlocks the queue mutex > > queue the buffer, set buffer > > state to queue > > > > queue the buffer, set buffer > > state to queue > > > > We would thus end up queueing the buffer twice. The vb->state check needs > > to be performed after the brief release of the queue mutex. > > Good point, I hadn't thought about that scenario. However, using mmap_sem to > introduce a large critical section just to protect against state changes is > IMHO not the right approach. Why not introduce a VB2_BUF_STATE_PREPARING > state? Note that we use the queue mutex to do so, not mmap_sem. The problem is that we can't release the queue mutex in the middle of a critical section without risking being preempted by another task. Introducing a new state might be possible if it effectively breaks the critical section in two independent parts. > That's set at the start of __buf_prepare while the queue mutex is still > held, and which prevents other threads of queuing the same buffer again. If > the prepare fails, then the state is reverted back to DEQUEUED. > > __fill_v4l2_buffer() will handle the PREPARING state as if it was the > DEQUEUED state. > > What do you think? I'll have to review that in details given the potential complexity of locking issues :-) I'm not opposed to the idea, if it works I believe we should do it. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
