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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1208190749.7375.10.camel@twins>
Date:	Mon, 14 Apr 2008 18:32:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Roland Dreier <rdreier@...co.com>
Cc:	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, 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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/