Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation

Thomas Gleixner
2005-12-16T22:35:55Z

On Fri, 2005-12-16 at 14:19 -0800, Linus Torvalds wrote:
>
> On Fri, 16 Dec 2005, Thomas Gleixner wrote:
> >
> > Well, in case of a semaphore it is a semantically correct use case. In
> > case of of a mutex it is not.
>
> I disagree.
>
> Think of "initialization" as a user. The system starts out initializing
> stuff, and as such the mutex should start out being held. It's that
> simple. It _is_ mutual exclusion, with one user being the early bootup
> state.

Mutual exclusion is available with various semantical characteristics.
If you want to have a particular semantical functionality you have to
chose a variant which fits that need. Arguing that the underlying
mechanism (implemenation) can handle your request is broken by
definition. It can, but it still is semantically wrong.

Mutexes have a well defined semantic of lock ownership, i.e. the thread
which locked a mutex has to unlock it. Semaphores do not have this
semantical requirement.

Therefor, if you want to handle that "init protection" scenario, do not
use a mutex, because the owner can not be defined at compile -
allocation time.

You can still implement (chose a mechanism) a mutex on top - or in case
of lack of priority inheritance or debugging with exactly the same -
mechanism as a semaphore, but this does not change the semantical
difference at all.

tglx