Re: [RFC PATCH 0/8] V4L BKL removal: first round

[Arnd Bergmann <arnd@arndb.de>]

On Sunday 14 November 2010 23:48:51 Hans Verkuil wrote:
> On Sunday, November 14, 2010 22:53:29 Arnd Bergmann wrote:
> > On Sunday 14 November 2010, Hans Verkuil wrote:
> > > This patch series converts 24 v4l drivers to unlocked_ioctl. These are low
> > > hanging fruit but you have to start somewhere :-)
> > > 
> > > The first patch replaces mutex_lock in the V4L2 core by mutex_lock_interruptible
> > > for most fops.
> > 
> > The patches all look good as far as I can tell, but I suppose the title is
> > obsolete now that the BKL has been replaced with a v4l-wide mutex, which
> > is what you are removing in the series.
> 
> I guess I have to rename it, even though strictly speaking the branch I'm
> working in doesn't have your patch merged yet.
> 
> BTW, replacing the BKL with a static mutex is rather scary: the BKL gives up
> the lock whenever you sleep, the mutex doesn't. Since sleeping is very common
> in V4L (calling VIDIOC_DQBUF will typically sleep while waiting for a new frame
> to arrive), this will make it impossible for another process to access any
> v4l2 device node while the ioctl is sleeping.
> 
> I am not sure whether that is what you intended. Or am I missing something?

I was aware that something like this could happen, but I apparently
misjudged how big the impact is. The general pattern for ioctls is that
those that get called frequently do not sleep, so it can almost always be
called with a mutex held.

	Arnd