[linux-dvb] [BUG] changeset 9029 (http://linuxtv.org/hg/v4l-dvb/rev/aa3e5cc1d833)

Andreas Oberritter obi at linuxtv.org
Tue Feb 17 05:02:21 CET 2009

Oliver Endriss wrote:
> Trent Piepho wrote:
>> On Sun, 15 Feb 2009, Oliver Endriss wrote:
>>> e9hack wrote:
>>>> this change set is wrong. The affected functions cannot be called from an interrupt
>>>> context, because they may process large buffers. In this case, interrupts are disabled for
>>>> a long time. Functions, like dvb_dmx_swfilter_packets(), could be called only from a
>>>> tasklet. This change set does hide some strong design bugs in dm1105.c and au0828-dvb.c.
>>>> Please revert this change set and do fix the bugs in dm1105.c and au0828-dvb.c (and other
>>>> files).
>>> @Mauro:
>>> This changeset _must_ be reverted! It breaks all kernels since 2.6.27
>>> for applications which use DVB and require a low interrupt latency.
>>> It is a very bad idea to call the demuxer to process data buffers with
>>> interrupts disabled!
>> I agree, this is bad.  The demuxer is far too much work to be done with
>> IRQs off.  IMHO, even doing it under a spin-lock is excessive.  It should
>> be a mutex.  Drivers should use a work-queue to feed the demuxer.
> Agreed, this would be the best solution.
> On the other hand, a workqueue handler would be scheduled later, so you
> need larger buffers in the driver. Some chipsets have very small
> buffers...
> Anway, this would be a major change. All drivers must be carefully
> modified and tested for an extended period.
> Meanwhile I had a look at the changeset, and I do not understand why
> spin_lock_irq... should be required everywhere.
> Afaics a driver may safely call dvb_dmx_swfilter_packets,
> dvb_dmx_swfilter_204 or dvb_dmx_swfilter from process context, tasklet
> or interrupt handler 'as is'.
> @Andreas:
> Could you please explain in more detail what bad things might happen?

To quote myself from the changelog: This fixes a deadlock discovered
by lockdep.

The lock is used in process context (e.g. DMX_START) and might also be
used from interrupt context (e.g. dvb_dmx_swfilter).

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