Re: [BUG] futex: scheduling-while-atomic because nested vfork can break guard(private_hash)

[Davidlohr Bueso]

On Fri, 11 Sep 2026, Jann Horn wrote:

> On Fri, Sep 11, 2026 at 1:51???PM Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
> > On Fri, Sep 11 2026 at 11:04, Peter Zijlstra wrote:
> > > Per commit: ee9dce44362b ("futex: Drop CLONE_THREAD requirement for private default hash alloc")
> > > the reason for excluding vfork() was performance and thinking this
> > > would/could not matter, which you've proven to be clearly false.
> > >
> > > Thomas?
> >
> > I don't remember why we excluded VFORK in the first place. Sebastian?
>
> That was introduced in commit ee9dce44362b ("futex: Drop CLONE_THREAD
> requirement for private default hash alloc"), and the commit message
> says:
>
> | Loosen the check to cover any CLONE_VM clone, except vfork().  Excluding
> | vfork keeps the existing paths untouched (no overhead), and we can't
> | race in the first place: either the parent is suspended and the child
> | runs alone, or mm->futex_ref is already allocated from an earlier
> | CLONE_VM.
>
> So I think that was purely supposed to be a performance optimization
> for single-threaded processes that do vfork()+exec()?

Right, but didn't really measure anything and am certainly fine with
Peter's fixlet. Sorry for breaking things.

Thanks,
Davidlohr