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

[Jason Gunthorpe]

On Tue, Sep 06, 2022 at 03:57:30PM +0200, David Hildenbrand wrote:

> > 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.

Yes, it is important, but it is not the only important part.

The compiler still has to exectute "if (*a != b)" *correctly*.

This is what READ_ONCE is for. It doesn't set order, it doesn't
implement a barrier, it tells the compiler that '*a' is unstable data
and the compiler cannot make assumptions based on the idea that
reading '*a' multiple times will always return the same value.

If the compiler makes those assumptions then maybe even though 'if (*a
!= b)' is the reality, it could mis-compute '*a == b'. You enter into
undefined behavior here.

Though it is all very unlikely, the general memory model standard is
to annotate with READ_ONCE.

Jason