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

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Date:	Tue, 27 May 2008 15:59:42 -0300
From:	Mauro Carvalho Chehab <mchehab@...radead.org>
To:	Jonathan Corbet <corbet@....net>
Cc:	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 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.

Cheers,
Mauro
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