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

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Date:	Tue, 15 Apr 2008 19:15:11 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Bart Van Assche <bart.vanassche@...il.com>,
	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
Subject: Re: [PATCH] Replace completions with semaphores

Linus Torvalds wrote:
> 
> On Tue, 15 Apr 2008, Andi Kleen wrote:
>>>  - probably add support for completions to do counting
>> But that's just a semaphore, isn't it?
> 
> Exactly. But the point here is:
> 
>  - nobody should use semaphores anyway (use mutexes)

For normal locks. But if you have N number of outstanding
events you need to wait for the semaphore is the right primitive.

And it seems there is a not high but non trivial number
of places in the kernel who have a legitimate need for this.

>  - making *more* code use semaphores is wrong
>  - completions have a different _mental_ model
>
> IOW, this is not about implementation issues. It's about how you think 
> about the operations.

Ok so you just want to rename it.

Fine for me. I always found up() and down() unintuitive anyways
(but it's admittedly better than "P" and "V" which some other systems use)

> We should _not_ implement completions as semaphores, simply because we 
> want to get *rid* of semaphores some day. 
> 
> So rather than this long and involved patch series that first makes 
> semaphores generic, and then makes them be used as completions, I'd much 
> rather just skip this whole pointless exercise entirely. 
> 
> Why have "generic semaphores" at all, if we want to get rid of them? 

Because we still "counted completions" for some things and that's the
same code?

Rather i suspect the real problem is not the name, but just not sure
it gets abused. That is largely more a review problem and as far as I
can figure out basically all the usual reviewers take care of that
anyways. But renaming it also probably wouldn't hurt.

[IMHO I always thought we should have a maintained single "list of
things for reviewers to watch out for" list somewhere]

-Andi

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