Re: videobuf lock problem

[Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxx>]

Hi Hans,

On Wed, 30 Jan 2008 11:52:40 +0100 (CET)
"Hans Verkuil" <hverkuil@xxxxxxxxx> wrote:

> Hi all,
> 
> Am I missing something, or is there indeed a deadlock bug in videobuf-core?
> 
> At work we are using one thread that dequeues buffers from a v4l output
> device and another thread that queues the filled buffers into the same
> output device. However, videobuf_dqbuf() locks q->lock at the start and
> never releases it until it gets the buffer. If there are no buffers
> available, then it will wait without releasing the mutex! But
> videobuf_qbuf() also tries to lock q->lock. So videobuf_dqbuf is waiting
> for a new buffer to become available, but videobuf_qbuf cannot queue a
> buffer become videobuf_dqbuf still has the mutex locked.
> 
> I'm no videobuf expert, but I'd say that videobuf_dqbuf should release the
> q->lock mutex when calling videobuf_waiton(). This is probably true for
> all calls to videobuf_waiton().
> 
> I just hacked that in and it seems to work now (I just added an unlock
> before the waiton() and a lock afterwards). But I don't know if there are
> any subtleties that I'm missing.
> 
> Mauro, can you take a look at this?

Videobuf is a very complex piece of software. It supports several different
operation modes. Also, some boards touch on videobuf priv locks.

I can't foresee any troubles with your approach, except for the fact that
waiton() may be called externally. In fact, bttv, cx23885, saa7134, saa7146 and
vivi calls waiton. This happens at interrupt time, when a buffer is filled.

So, if you simply do mutex_unlock() inside waiton(), you're doing something bad.

I'll take a look for a proper solution. What's the driver you're using? What
mode (read, userspace memory allocated mmap, kernelspace memory allocated mmap)?


> 
> Thanks,
> 
>          Hans
> 




Cheers,
Mauro

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