Re: [RFC PATCH 1/8] vb2: push the mmap semaphore down to __buf_prepare()

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Wednesday 27 November 2013 08:12:24 Hans Verkuil wrote:
> On 11/26/2013 04:42 PM, Laurent Pinchart wrote:
> > 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@cisco.com>
> >>>> 
> >>>> 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?

As the devil is in the details I'd prefer a patch. I would have to write one 
to think about this anyway :-)

-- 
Regards,

Laurent Pinchart