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

[Linus Torvalds <torvalds@osdl.org>]

On Fri, 16 Dec 2005, Joe Korty wrote:
> 
> The Mars Pathfinder incident is sufficient proof that some solution to
> the priority inversion problem is required in real systems.

Ehh. 

The Mars Pathfinder is just about the worst case "real system", and if I 
recall correctly, the reason it was able to continue was _not_ because it 
handled priority inversion, but because it reset itself every 24 hours or 
something like that, and had debugging facilities..

The _real_ lesson you should take away from it is not that priority 
inheritance is a good solution to priority inversion, but that having a 
failsafe switch when everthing goes wrong is critical. You don't know 
_what_ bug you'll encounter.

The bug itself could have been solved without priority inheritance, 
although I think in this case enabling that in VxWorks was the particular 
solution to the problem as being the least invasive.

Personally, I don't care what user space does. If some app wants to use 
priority inheritance to solve its bugs, that's fine. But it's like 
recursive locks: it's generally a _bandaid_ for bad locking. I definitely 
don't want the kernel depending on either.

So put a watchdog on your critical systems, and make sure you can debug 
them. Especially if they're on Mars.

			Linus
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