Re: [PATCH v3 tip/perf/core 4/4] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

[Peter Zijlstra]

On Wed, Oct 23, 2024 at 01:02:53PM -0700, Andrii Nakryiko wrote:

> > > +      * but can't be freed from under us, so it's safe to read fields from
> > > +      * it, even if the values are some garbage values; ultimately
> > > +      * find_uprobe_rcu() + mmap_lock_speculation_end() check will ensure
> > > +      * that whatever we speculatively found is correct
> > > +      */
> > > +     vm_file = READ_ONCE(vma->vm_file);
> > > +     if (!vm_file)
> > > +             return NULL;
> > > +
> > > +     vm_pgoff = data_race(vma->vm_pgoff);
> > > +     vm_start = data_race(vma->vm_start);
> > > +     vm_inode = data_race(vm_file->f_inode);
> >
> > So... seqcount has kcsan annotations other than data_race(). I suppose
> > this works, but it all feels like a bad copy with random changes.
> 
> I'm not sure what this means... Do I need to change anything? Drop
> data_race()? Use READ_ONCE()? Do nothing?

Keep for now. I've ranted at 1/n a bit, but unless the response is:
yeah, obviously this should be seqcount (unlikely) this is something
that can be fixed later (*sigh* always later... this todo list is a
problem).