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

[Andi Kleen]

> 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. 

Sure you can probably do better, but that would involve
patching all architectures won't it? Ok I suppose
you could make an ARCH_HAS_blabla white list, but that
wouldn't be exactly pretty.

-Andi
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