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

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Date:	Wed, 7 Jan 2009 13:47:15 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	torvalds@...ux-foundation.org, matthew@....cx, rostedt@...dmis.org,
	peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
	ghaskins@...ell.com, andi@...stfloor.org, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, tglx@...utronix.de, npiggin@...e.de,
	pmorreale@...ell.com, SDietrich@...ell.com
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

On Wed, 7 Jan 2009 22:32:22 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> > 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

tisn't.  pagefault_disable() is just preempt_count()+=1;barrier() ?

Am suspecting that you guys might be over-optimising this
contended-path-were-going-to-spin-anyway code?
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