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

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Message-ID: <adafxtoqvgh.fsf@cisco.com>
Date:	Mon, 14 Apr 2008 08:58:54 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Matthew Wilcox <matthew@....cx>,
	Peter Zijlstra <peterz@...radead.org>,
	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

 > 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.

 - R.
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