Messages in this thread | | | Date | Tue, 27 May 2008 17:24:46 -0300 | From | Mauro Carvalho Chehab <> | Subject | Re: [PATCH] video4linux: Push down the BKL |
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On Tue, 27 May 2008 12:50:41 -0700 Arjan van de Ven <arjan@infradead.org> wrote:
> On Tue, 27 May 2008 15:59:42 -0300 > Mauro Carvalho Chehab <mchehab@infradead.org> wrote: > > > On Tue, 27 May 2008 10:37:55 -0600 > > Jonathan Corbet <corbet@lwn.net> wrote: > > > > > On Tue, 27 May 2008 13:31:00 -0300 > > > Mauro Carvalho Chehab <mchehab@infradead.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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