Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

[David Hildenbrand]

On 06.09.22 15:47, Jason Gunthorpe wrote:
> On Mon, Sep 05, 2022 at 09:59:47AM +0200, David Hildenbrand wrote:
>
> > > That should be READ_ONCE() for the *pmdp and *ptep reads. Because this
> > > whole lockless house of cards may fall apart if we try reading the
> > > page table values without READ_ONCE().
> >
> > I came to the conclusion that the implicit memory barrier when grabbing a
> > reference on the page is sufficient such that we don't need READ_ONCE here.
>
> READ_ONCE is not about barriers or ordering, you still need the
> acquire inside the atomic to make the algorithm work.


While I don't disagree with what say is, I'll refer to 
Documentation/memory-barriers.txt "COMPILER BARRIER".

As discussed somewhere in this thread, if we already have an atomic RWM 
that implies a full ordering, it implies a compile barrier.


> READ_ONCE primarily is a marker that the data being read is unstable
> and that the compiler must avoid all instability when reading it. eg
> in this case the compiler could insanely double read the value, even
> though the 'if' requires only a single read. This would result in
> corrupt calculation.

As we have a full memory barrier + compile barrier, the compiler might 
indeed do double reads and all that stuff. BUT, it has to re-read after 
we incremented the refcount, and IMHO that's the important part to 
detect the change.



--
Thanks,

David / dhildenb