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

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Date:	Mon, 14 Apr 2008 09:56:38 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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 Mon, 14 Apr 2008 18:32:28 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, 2008-04-14 at 08:58 -0700, Roland Dreier wrote:
> > > which ones exactly are these places that demand the use of a
> > > counting 
> >  > semaphore? I cannot think of a single place where it's the best
> >  > choice, let alone one where it's the only choice.
> > 
> > Two of the places that use semaphores are
> > drivers/infiniband/hw/mthca and drivers/net/mlx4 -- in both cases,
> > the device firmware allows up to "N" outstanding firmware commands
> > to be in flight, and the driver uses a semaphore to handle issuing
> > firmware commands.  That is, down() when we want to issue a
> > command, and up() when the firmware responds that the command is
> > complete.
> > 
> > What would you suggest as a better way to code this?  This is an
> > honest question -- there probably is a more elegant way to handle
> > this situation and I really would like to learn about it.
> > 
> > Also, the argument that removing semaphores makes the kernel as a
> > whole better does make sense to me; I wouldn't be opposed to
> > basically open-coding semaphores in terms of wait_event() in the
> > driver or something like that, but I wouldn't say that such an
> > implementation is locally more readable or maintainable if we look
> > only at the driver code.
> 
> Yeah, I would open code it.  But this is indeed a sane usage of the
> counting semaphore because there is no priority inversion.

Maybe we need a "counter" primitive instead?
>From a conceptual point of view that even makes sense

(the implementation can be pretty much the current semaphore one of course)

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