linux-kernel - Re: [PATCH] video4linux: Push down the BKL

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Date:	Tue, 27 May 2008 17:24:46 -0300
From:	Mauro Carvalho Chehab <mchehab@...radead.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Jonathan Corbet <corbet@....net>, Alan Cox <alan@...hat.com>,
	video4linux-list@...hat.com, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] video4linux: Push down the BKL

On Tue, 27 May 2008 12:50:41 -0700
Arjan van de Ven <arjan@...radead.org> wrote:

> On Tue, 27 May 2008 15:59:42 -0300
> Mauro Carvalho Chehab <mchehab@...radead.org> wrote:
> 
> > On Tue, 27 May 2008 10:37:55 -0600
> > Jonathan Corbet <corbet@....net> wrote:
> > 
> > > On Tue, 27 May 2008 13:31:00 -0300
> > > Mauro Carvalho Chehab <mchehab@...radead.org> wrote:
> > > 
> > > > Since the other methods don't explicitly call BKL (and, AFAIK,
> > > > kernel open handler don't call it neither), if a program 1 is
> > > > opening a device and initializing some data, and a program 2
> > > > starts doing ioctl, interrupting program 1 execution in the
> > > > middle of a data initialization procedure, you may have a race
> > > > condition, since some devices initialize some device global data
> > > > during open [1].
> > > 
> > > In fact, 2.6.26 and prior kernels *do* acquire the BKL on open (for
> > > char devices) - that's the behavior that the bkl-removal tree is
> > > there to do away with.  So, for example, I've pushed that
> > > acquisition down into video_open() instead. 
> > > 
> > > So, for now, open() is serialized against ioctl() in video
> > > drivers.  As soon as you take the BKL out of ioctl(), though, that
> > > won't happen, unless the mutex you use is also acquired in the open
> > > path.
> > 
> > Ok.
> > 
> > A few drivers seem to be almost read to work without BKL. 
> > 
> > For example, em28xx has already a lock at the operations that change
> > values at "dev" struct, including open() method. However, since the
> > lock is not called at get operations, it needs to be fixed. I would
> > also change it from mutex to a read/write semaphore, since two (or
> > more) get operations can safely happen in parallel.
> > 
> \
> 
> please don't use rw/sems just because there MIGHT be parallel.
> THey're more expensive than mutexes by quite a bit and you get a lot
> less checking from lockdep. They make sense for very specific, very
> read biased, contended cases. But please don't use them "just
> because"...
> 
Good point. The nature of get operations on V4L are not worthy enough to justify
the loss of lockdep checking.

Cheers,
Mauro
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