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

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Message-ID: <20090107213222.GE4597@elte.hu>
Date:	Wed, 7 Jan 2009 22:32:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> 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).

for the oldfs stuff we already have probe_kernel_read(). OTOH, that 
involves pagefault_disable() which is an atomic op, so 
__get_user_careful() should be much more lightweight - and we already know 
that the memory range at least _used to_ be a valid kernel address.

(Theoretical race: with memory hotplug that kernel pointer address could 
have gotten unmapped and we could get device memory there - with 
side-effects if accessed. Wont happen in practice.)

	Ingo
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