From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning
Date: Wed, 7 Jan 2009 13:35:35 -0800
Message-ID: <20090107133535.858ce125.akpm@linux-foundation.org>
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Cc: torvalds@linux-foundation.org, peterz@infradead.org,
	paulmck@linux.vnet.ibm.com, ghaskins@novell.com, mingo@elte.hu,
	matthew@wil.cx, andi@firstfloor.org, chris.mason@oracle.com,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-btrfs@vger.kernel.org, tglx@linutronix.de,
	rostedt@goodmis.org, npiggin@suse.de, pmorreale@novell.com,
	SDietrich@novell.com
To: Andi Kleen <andi@firstfloor.org>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
In-Reply-To: <20090107213740.GO496@one.firstfloor.org>
List-ID: <linux-btrfs.vger.kernel.org>

On Wed, 7 Jan 2009 22:37:40 +0100
Andi Kleen <andi@firstfloor.org> wrote:

> > But we can do that with __get_user(thread_info->cpu) (very unlikely page 
> > fault protection due to the possibility of CONFIG_DEBUG_PAGEALLOC) and 
> > then validating the cpu. It it's in range, we can use it and verify 
> > whether cpu_rq(cpu)->curr has that thread_info.
> > 
> > So we can do all that locklessly and optimistically, just going back and 
> > verifying the results later. This is why "thread_info" is actually a 
> > better thing to use than "task_struct" - we can look up the cpu in it with 
> > a simple dereference. We knew the pointer _used_ to be valid, so in any 
> > normal situation, it will never page fault (and if you have 
> > CONFIG_DEBUG_PAGEALLOC and hit a very unlucky race, then performance isn't 
> > your concern anyway: we just need to make the page fault be non-lethal ;)
> 
> The problem with probe_kernel_address() is that it does lots of
> operations around the access in the hot path (set_fs, pagefault_disable etc.), 
> so i'm not sure that's a good idea. 

probe_kernel_address() isn't tooooo bad - a few reads and writes into
the task_struct and thread_struct.  And we're on the slow, contended
path here anyway..