Re: [PATCH v2 tip/perf/core 5/5] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Oct 2, 2024 at 12:25 AM Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 10/01, Andrii Nakryiko wrote:
> >
> > +static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
> > +{
> > +     struct mm_struct *mm = current->mm;
> > +     struct uprobe *uprobe = NULL;
> > +     struct vm_area_struct *vma;
> > +     struct file *vm_file;
> > +     loff_t offset;
> > +     long seq;
> > +
> > +     guard(rcu)();
> > +
> > +     if (!mmap_lock_speculation_start(mm, &seq))
> > +             return NULL;
> > +
> > +     vma = vma_lookup(mm, bp_vaddr);
> > +     if (!vma)
> > +             return NULL;
> > +
> > +     /* vm_file memory can be reused for another instance of struct file,
> > +      * 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;
> > +
> > +     offset = (loff_t)(vma->vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vma->vm_start);
>
> LGTM. But perhaps vma->vm_pgoff and vma->vm_start need READ_ONCE() as well,
> if nothing else to shut up KCSAN if this code races with, say, __split_vma() ?

We keep going back and forth between reading directly, using
READ_ONCE(), and annotating with data_race(). I don't think it matters
in terms of correctness or performance, so I'm happy to add whatever
incantations that will make everyone satisfied. Let's see what others
think, and I'll incorporate that into the next revision.

>
> > +     uprobe = find_uprobe_rcu(vm_file->f_inode, offset);
>
> OK, I guess vm_file->f_inode is fine without READ_ONCE...
>
> Oleg.
>