Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Peter Zijlstra]

On Wed, Jan 15, 2025 at 11:48:41AM +0100, Peter Zijlstra wrote:
> On Sat, Jan 11, 2025 at 12:14:47PM -0800, Suren Baghdasaryan wrote:
> 
> > > Replacing down_read_trylock() with the new routine loses an acquire
> > > fence. That alone is not a problem, but see below.
> > 
> > Hmm. I think this acquire fence is actually necessary. We don't want
> > the later vm_lock_seq check to be reordered and happen before we take
> > the refcount. Otherwise this might happen:
> > 
> > reader             writer
> > if (vm_lock_seq == mm_lock_seq) // check got reordered
> >         return false;
> >                        vm_refcnt += VMA_LOCK_OFFSET
> >                        vm_lock_seq == mm_lock_seq
> >                        vm_refcnt -= VMA_LOCK_OFFSET
> > if (!__refcount_inc_not_zero_limited())
> >         return false;
> > 
> > Both reader's checks will pass and the reader would read-lock a vma
> > that was write-locked.
> 
> Hmm, you're right. That acquire does matter here.

Notably, it means refcount_t is entirely unsuitable for anything
SLAB_TYPESAFE_BY_RCU, since they all will need secondary validation
conditions after the refcount succeeds.

And this is probably fine, but let me ponder this all a little more.