linux-kernel - Re: [PATCH] Replace completions with semaphores

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Date:	Tue, 15 Apr 2008 09:17:09 +0200
From:	"Bart Van Assche" <bart.vanassche@...il.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"Andi Kleen" <andi@...stfloor.org>,
	"Roland Dreier" <rdreier@...co.com>, "Ingo Molnar" <mingo@...e.hu>,
	"Matthew Wilcox" <matthew@....cx>,
	"Ingo Oeser" <ioe-lkml@...eria.de>,
	"Daniel Walker" <dwalker@...sta.com>, linux-kernel@...r.kernel.org,
	"Linus Torvalds" <torvalds@...l.org>
Subject: Re: [PATCH] Replace completions with semaphores

On Tue, Apr 15, 2008 at 8:46 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, 2008-04-15 at 08:18 +0200, Bart Van Assche wrote:
>  > Would it really be a good idea to give a synchronization concept that
>  > behaves exactly like a semaphore another name than "semaphore" ? The
>  > semaphore concept is well known and is taught in every computer
>  > science course.
>
>  Are the ramifications wrt priority inversion taught? Is it made clear
>  that its hard to validate because there is no clear resource owner?
>
>  Afaik, non of these subjects are touched upon in the CS-101 courses and
>  that is exactly the problem. So you can say they are not well know, they
>  are just widely misunderstood.
>
>  And yes, if there are more hand a very few such users it doesn't make
>  sense to keep them open coded.

Regarding semaphores and priority inversion: I have never recommended
the use of semaphores over mutexes, all I recommended is to keep the
name "semaphore" for something that behaves like a semaphore. There
might be better ways to discourage the use of the semaphore API, e.g.
letting the compiler print a warning every time a semaphore function
is called unless one or another #define has been enabled.

Regarding priority inheritance: does the above mean that you consider
priority inheritance as an optimal solution for realizing real-time
behavior in the kernel ? Are you aware of the fundamental problems
associated with priority inheritance ? These issues are well explained
in Victor Yodaiken's paper "Against priority inheritance". See also
http://www.linuxdevices.com/files/misc/yodaiken-july02.pdf .

Bart.
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