Re: [PATCH] x86/mm/cpa: get rid of the cpa lock

[Ingo Molnar <mingo@kernel.org>]

* Jacky Li <jackyli@google.com> wrote:

> It’s true that with such old code, the cpa_lock might protect more
> race conditions than those that it was introduced to protect in 2008,
> or some old hardware may depend on the cpa_lock for undocumented
> behavior. So removing the lock directly might not be a good idea, but
> it probably should not mean that we need to keep the inefficient code
> forever. I would appreciate any suggestion to navigate this lock
> removal from the folks on the to and cc list.

> -/*
> - * Serialize cpa() (for !DEBUG_PAGEALLOC which uses large identity mappings)
> - * using cpa_lock. So that we don't allow any other cpu, with stale large tlb
> - * entries change the page attribute in parallel to some other cpu
> - * splitting a large page entry along with changing the attribute.
> - */
> -static DEFINE_SPINLOCK(cpa_lock);

Yeah, so I'm *really* tempted to just remove cpa_lock if there's no in-code 
documented uses of it - your patch provides *exhaustive* background.

The thing is, even in the worst-case if it breaks anything, it will get 
investigated, documented better and maybe reverted - which would *still* be 
an improvement over today, because we turn undocumented code into 
documented code.

We cannot indefinitely keep a global lock just because we fear it might 
have some undocumented dependencies...

But no strong feelings either way - I've added a few more Cc:s to discuss 
this more widely.

Unless there's objections I'd be inclined to give this patch a try, and 
keep an eye open for regressions, it's not difficult to revert either.

Thanks,

	Ingo