Re: [patch] Real-Time Preemption, -RT-2.6.9-rc4-mm1-U8

[Jens Axboe]

On Thu, Oct 21 2004, Thomas Gleixner wrote:
> On Thu, 2004-10-21 at 22:38, Bill Huey wrote:
> > On Thu, Oct 21, 2004 at 10:33:50PM +0200, Jens Axboe wrote:
> > > On Thu, Oct 21 2004, Bill Huey wrote:
> > > > You use a semaphore to protect data, a completion isn't protecting data
> > > > but preserving a certain kind of wait ordering in the code. The
> > > > possibility of overloading the current mutex_t for PI makes for a conceptual
> > > > mismatch when used in this case since having a kind of priority for
> > > > completions is a bit odd. It's better to flat out use a completion
> > > > instead, IMO.
> > > 
> > > Linux semaphores (being counted) have always been a fine fit for things
> > > like the loop use, where you get to down it 10 times because you have 10
> > > items pending. I know this isn't the traditional mutex and that it
> > > doesn't protect data as such, but is was never abuse. It isn't overload.
> > > Doing it with a traditional mutex (I'm assuming this is what mutex_t is
> > > in Ingos tree) would be overload and a bad idea, indeed.
> > 
> > Well, this is something that's got to be considered by the larger Linux
> > community and whether these conventions are to be kept or removed. It's
> > a larger issue than what can be address in Ingo's preemption patch, but
> > with inevitable need for something like this in the kernel (hard RT)
> > it's really unavoidable collision. IMO, it's got to go, which is a nasty
> > change.
> > 
> 
> Hey, let's stop this here.
> 
> You are both (in)correct :)
> 
> 1. It makes no sense to discuss, why X has been considered correct for
> time T.

Because it is correct. Debating that it's now incorrect because it
inconveniently happens to make some detection scheme harder is silly.

> 2. Counted semaphores are a valid use and should be marked explicit as
> counted semaphores.

Indeed

> 3. Using mutexes and semaphores for event and completion signalling
> should be converted to the appropriate interfaces. 

Agree. Do you test all your conversions? Whole-sale conversions like
this tend to break at least some of the drivers. And that's totally
unacceptable, breaking a working solution because of something that's
not really a bug.

> A bunch of work, but not really hard.

Not if you don't test it.

-- 
Jens Axboe

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