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

[Peter Zijlstra]

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.