Re: [PATCH 16/17] prmem: pratomic-long

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 30, 2018 at 04:28:16PM +0000, Will Deacon wrote:
> On Tue, Oct 30, 2018 at 04:58:41PM +0100, Peter Zijlstra wrote:
> > Like mentioned elsewhere; if you do write_enable() + write_disable()
> > thingies, it all becomes:
> > 
> > 	write_enable();
> > 	atomic_foo(&bar);
> > 	write_disable();
> > 
> > No magic gunk infested duplication at all. Of course, ideally you'd then
> > teach objtool about this (or a GCC plugin I suppose) to ensure any
> > enable reached a disable.
> 
> Isn't the issue here that we don't want to change the page tables for the
> mapping of &bar, but instead want to create a temporary writable alias
> at a random virtual address?
> 
> So you'd want:
> 
> 	wbar = write_enable(&bar);
> 	atomic_foo(wbar);
> 	write_disable(wbar);
> 
> which is probably better expressed as a map/unmap API. I suspect this
> would also be the only way to do things for cmpxchg() loops, where you
> want to create the mapping outside of the loop to minimise your time in
> the critical section.

Ah, so I was thikning that the altnerative mm would have stuff in the
same location, just RW instead of RO.

But yes, if we, like Andy suggets, use the userspace address range for
the aliases, then we need to do as you suggest.