linux-kernel - Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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Date:	Wed, 7 Jan 2009 13:24:24 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Matthew Wilcox <matthew@....cx>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning



On Wed, 7 Jan 2009, Matthew Wilcox wrote:
> 
> I appreciate this is sample code, but using __get_user() on
> non-userspace pointers messes up architectures which have separate
> user/kernel spaces (eg the old 4G/4G split for x86-32).  Do we have an
> appropriate function for kernel space pointers?  Is this a good reason
> to add one?

Yes, you're right. 

We could do the whole "oldfs = get_fs(); set_fs(KERNEL_DS); .. 
set_fs(oldfs);" crud, but it would probably be better to just add an 
architected accessor. Especially since it's going to generally just be a

	#define get_kernel_careful(val,p) __get_user(val,p)

for most architectures.

We've needed that before (and yes, we've simply mis-used __get_user() on 
x86 before rather than add it).

			Linus
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