Re: [PATCH] Priority Lists for the RT mutex

[Bill Huey (hui) <bhuey@lnxw.com>]

On Mon, Apr 11, 2005 at 04:28:25PM -0700, Perez-Gonzalez, Inaky wrote:
> >From: Bill Huey (hui) [mailto:bhuey@lnxw.com]
...
> API than once upon a time was made multithreaded by just adding
> a bunch of pthread_mutex_[un]lock() at the API entry point...
> without realizing that some of the top level API calls also 
> called other top level API calls, so they'd deadlock.

Oh crap.

> Quick fix: the usual. Enable deadlock detection and if it
> returns deadlock, assume it is locked already and proceed (or
> do a recursive mutex, or a trylock).

You have to be joking me ? geez.
... 
> It is certainly something to explore, but I'd better drive your
> way than do it. It's cleaner. Hides implementation details.
>
> I agree, but it doesn't work that well when talking about legacy 
> systems...that's the problem.

Yeah, ok, I understand what's going on now. There isn't a notion
of projecting priority across into the Unix/Linux kernel traditionally
which is why it seemed so bizarre.

> Sure--and because most was for legacy reasons that adhered to 
> POSIX strictly, it was very simple: we need POSIX this, that and
> that (PI, proper adherence to scheduler policy wake up/rt-behaviour,
> deadlock detection, etc). 

Some of this stuff sounds like recursive locking. Would this be a
better expression to solve the "top level API locking" problem
you're referring to ?

> Fortunately in those areas POSIX is not too gray; code to the book.
> Deal. 

I would think that there will have to be a graph discontinuity
between user/kernel spaces at kernel entry and exit for the deadlock
detector. Can't say about issues at fork time, but I would expect
that those objects would have to be destroyed when the process exits.

The current RT (Ingo's) lock isn't recursive nor is the deadlock
detector the last time I looked. Do think that this is a problem
for legacy apps if it gets overload for being the userspace futex
as well ? (assuming I'm understanding all of this correctly)

> Of course, selling it to the lkml is another story.

I would think that pushing as much of this into userspace would
make the kernel hooks for it more acceptable. Don't know.

/me thinks more

bill