Re: [PATCH v2 2/3] mutex: add support for reservation style locks, v2

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Wed, Apr 10, 2013 at 12:27 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu, Apr 04, 2013 at 06:38:36PM +0200, Peter Zijlstra wrote:
>> On Thu, 2013-04-04 at 15:31 +0200, Daniel Vetter wrote:
>> > Hm, I guess your aim with the TASK_DEADLOCK wakeup is to bound the
>> > wait
>> > times of older task.
>>
>> No, imagine the following:
>>
>> struct ww_mutex A, B;
>> struct mutex C;
>>
>>       task-O  task-Y  task-X
>>               A
>>               B
>>                       C
>>               C
>>       B
>>
>> At this point O finds that Y owns B and thus we want to make Y 'yield'
>> B to make allow B progress. Since Y is blocked, we'll send a wakeup.
>> However Y is blocked on a different locking primitive; one that doesn't
>> collaborate in the -EDEADLK scheme therefore we don't want the wakeup to
>> succeed.
>
> I'm confused to why the above is a problem. Task-X will eventually
> release C, and then Y will release B and O will get to continue. Do we
> have to drop them once the owner is blocked? Can't we follow the chain
> like the PI code does?

Just waiting until every task already holding a lock completes and
unlucks it is indeed a viable solution - it's the currently
implemented algorithm in ttm and Maarten's current patches.

The nice thing with Peter's wakeup idea on top is:
- It bounds blocked times.
- And (at least I think so) it's the key thing making PI boosting
possible without any ugly PI inversion deadlocks happening. See

Message-ID: <CAKMK7uEUdtiDDCRPwpiumkrST6suFY7YuQcPAXR_nJ0XHKzsAw@mail.gmail.com>

for my current reasoning about this (I have not yet managed to poke a
hole into it).
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch