Re: [RFC PATCH 2/8] hugetlb: recompute min_count when dropping hugetlb_lock

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 23, 2021 at 08:50:53AM +0100, Michal Hocko wrote:

> > >> +static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
> > >> +{
> > >> +	unsigned long min_count;
> > >> +
> > >> +	min_count = h->resv_huge_pages + h->nr_huge_pages - h->free_huge_pages;
> > >> +	return max(count, min_count);
> > > 
> > > Just out of curiousity, is compiler allowed to inline this piece of code
> > > and then cache the value? In other words do we need to make these
> > > READ_ONCE or otherwise enforce the no-caching behavior?
> > 
> > I honestly do not know if the compiler is allowed to do that.  The
> > assembly code generated by my compiler does not cache the value, but
> > that does not guarantee anything.  I can add READ_ONCE to make the
> > function look something like:
> > 
> > static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
> > {
> > 	unsigned long min_count;
> > 
> > 	min_count = READ_ONCE(h->resv_huge_pages) + READ_ONCE(h->nr_huge_pages)
> > 					- READ_ONCE(h->free_huge_pages);
> > 	return max(count, min_count);
> > }
> 
> Maybe just forcing to never inline the function should be sufficient.
> This is not a hot path to micro optimize for no function call. But there
> are much more qualified people on the CC list on this matter who could
> clarify. Peter?

I'm not sure I understand the code right. But inline or not doesn't
matter, LTO completely ruins that game. Just like if it was a static
function, then the compiler is free to inline it, even if the function
lacks an inline attribute.

Basically, without READ_ONCE() the compiler is allowed to entirely elide
the load (and use a previous load), or to duplicate the load and do it
again later (reaching a different result).

Similarly, the compiler is allowed to byte-wise load the variable in any
random order and re-assemble.

If any of that is a problem, you have to use READ_ONCE().