On 11/26/2013 04:42 PM, Laurent Pinchart wrote: > 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. > Do you want to think about this first, or shall I make a new patch that you can then review? Regards, Hans -- 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
