Re: [RFC PATCH 08/11] mm/hmm: Use lockdep instead of comments

[Souptick Joarder <jrdr.linux@gmail.com>]

On Sat, Jun 8, 2019 at 1:09 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Sat, Jun 08, 2019 at 01:03:48AM +0530, Souptick Joarder wrote:
> > On Thu, May 23, 2019 at 9:05 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > From: Jason Gunthorpe <jgg@mellanox.com>
> > >
> > > So we can check locking at runtime.
> > >
> > > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> > >  mm/hmm.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/hmm.c b/mm/hmm.c
> > > index 2695925c0c5927..46872306f922bb 100644
> > > +++ b/mm/hmm.c
> > > @@ -256,11 +256,11 @@ static const struct mmu_notifier_ops hmm_mmu_notifier_ops = {
> > >   *
> > >   * To start mirroring a process address space, the device driver must register
> > >   * an HMM mirror struct.
> > > - *
> > > - * THE mm->mmap_sem MUST BE HELD IN WRITE MODE !
> > >   */
> > >  int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
> > >  {
> > > +       lockdep_assert_held_exclusive(mm->mmap_sem);
> > > +
> >
> > Gentle query, does the same required in hmm_mirror_unregister() ?
>
> No.. The unregistration path does its actual work in the srcu
> callback, which is in a different context than this function. So any
> locking held by the caller of unregister will not apply.
>
> The hmm_range_free SRCU callback obtains the write side of mmap_sem to
> protect the same data that the write side above in register is
> touching, mostly &mm->hmm.

Looking into https://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma.git/tree/?h=hmm,
unable trace hmm_range_free(). Am I looking into correct tree ?