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

[Andrew Morton <akpm@linux-foundation.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..