[linux-dvb] [PATCH] dvb-core: Fix several locking related problems.

Simon Arlott simon at fire.lp0.eu
Mon Mar 5 13:13:59 CET 2007


On Mon, March 5, 2007 11:19, Johannes Stezenbach wrote:
> On Mon, Mar 05, 2007 at 01:58:14AM +0100, Oliver Endriss wrote:
>> Simon Arlott wrote:
>> > Is any part of the patch going to be applied? I mentioned this
>> > problem in September last year and it looks like it's existed for
>> > years (the semaphore locking did the same thing).
>>
>> Well, I hoped that someone more familiar with the demuxer stuff would
>> comment on the patch. I am not very happy about using non-interruptible
>> lock operations...

Why? If there are deadlocks these should be fixed, not ignored.

>> Anyway, I'll apply the patch to HG master if you submit an updated patch:
>> - Please add a line of comment to each mutex_lock() stating _why_ the
>>   non-interruptible lock has to be used at this place.

What's the point of doing that?

> IMHO using mutex_lock_interruptible() is almost always wrong.
>
> The only use it has in dvb-core is to recover from locking bugs --
> if it deadlocks you can Ctrl-C out of it
> (instead of being left with a non-killable program -> reboot).

This is what lockdep is for.

> But with mutex_lock_interruptible() it's easy to introduce
> signal handling bugs, which Simon confirmed to exist.

It's also easy to find examples of people needing to rmmod/modprobe because dvr0 started returning -EBUSY on
open() after they closed something.

> Fixing those up without reverting to mutex_lock() way might
> be possible, but is more difficult.

It'd introduce lot of unneccessary -ERESTARTSYS, just to avoid the possiblity of waiting on mutex_lock for a
few msecs.

-- 
Simon Arlott



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