linux-kernel - Re: [PATCH 0/5] Pushdown bkl from v4l ioctls

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Message-Id: <201005011155.37057.hverkuil@xs4all.nl>
Date:	Sat, 1 May 2010 11:55:37 +0200
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, John Kacur <jkacur@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Blunck <jblunck@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Greg KH <gregkh@...e.de>,
	Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH 0/5] Pushdown bkl from v4l ioctls

On Thursday 29 April 2010 09:10:42 Laurent Pinchart wrote:
> Hi Hans,
> 
> On Thursday 29 April 2010 08:44:29 Hans Verkuil wrote:
> > On Thursday 29 April 2010 05:42:39 Frederic Weisbecker wrote:
> > > Hi,
> > > 
> > > Linus suggested to rename struct v4l2_file_operations::ioctl
> > > into bkl_ioctl to eventually get something greppable and make
> > > its background explicit.
> > > 
> > > While at it I thought it could be a good idea to just pushdown
> > > the bkl to every v4l drivers that have an .ioctl, so that we
> > > actually remove struct v4l2_file_operations::ioctl for good.
> > > 
> > > It passed make allyesconfig on sparc.
> > > Please tell me what you think.
> > 
> > I much prefer to keep the bkl inside the v4l2 core. One reason is that I
> > think that we can replace the bkl in the core with a mutex. Still not
> > ideal of course, so the next step will be to implement proper locking in
> > each driver. For this some additional v4l infrastructure work needs to be
> > done. I couldn't proceed with that until the v4l events API patches went
> > in, and that happened yesterday.
> > 
> > So from my point of view the timeline is this:
> > 
> > 1) I do the infrastructure work this weekend. This will make it much easier
> > to convert drivers to do proper locking. And it will also simplify
> > v4l2_priority handling, so I'm killing two birds with one stone :-)
> > 
> > 2) Wait until Arnd's patch gets merged that pushes the bkl down to
> > v4l2-dev.c
> > 
> > 3) Investigate what needs to be done to replace the bkl with a v4l2-dev.c
> > global mutex. Those drivers that call the bkl themselves should probably be
> > converted to do proper locking, but there are only about 14 drivers that do
> > this. The other 60 or so drivers should work fine if a v4l2-dev global lock
> > is used. At this point the bkl is effectively removed from the v4l
> > subsystem.
> > 
> > 4) Work on the remaining 60 drivers to do proper locking and get rid of the
> > v4l2-dev global lock. This is probably less work than it sounds.
> > 
> > Since your patch moves everything down to the driver level it will actually
> > make this work harder rather than easier. And it touches almost all drivers
> > as well.
> 
> Every driver will need to be carefully checked to make sure the BKL can be 
> replaced by a v4l2-dev global mutex. Why would it be more difficult to do so 
> if the BKL is pushed down to the drivers ?

The main reason is really that pushing the bkl into the v4l core makes it
easier to review. I noticed for example that this patch series forgot to change
the video_ioctl2 call in ivtv-ioctl.c to video_ioctl2_unlocked. And there may
be other places as well that were missed. Having so many drivers changed also
means a lot of careful reviewing.

But I will not block this change. However, I do think it would be better to
create a video_ioctl2_bkl rather than add a video_ioctl2_unlocked. The current
video_ioctl2 function *is* already unlocked. So you are subtle changing the
behavior of video_ioctl2. Not a good idea IMHO. And yes, grepping for
video_ioctl2_bkl is also easy to do and makes it more obvious that the BKL is
used in drivers that call this.

Regards,

	Hans

-- 
Hans Verkuil - video4linux developer - sponsored by TANDBERG, part of Cisco
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