linux-kernel - Re: [PATCH v4 0/1] Use HMM for ODP v4

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Date:   Thu, 23 May 2019 16:47:29 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Leon Romanovsky <leonro@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        Artemy Kovalyov <artemyko@...lanox.com>,
        Moni Shoua <monis@...lanox.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>
Subject: Re: [PATCH v4 0/1] Use HMM for ODP v4

On Thu, May 23, 2019 at 03:39:59PM -0400, Jerome Glisse wrote:
> On Thu, May 23, 2019 at 04:10:38PM -0300, Jason Gunthorpe wrote:
> > 
> > On Thu, May 23, 2019 at 02:24:58PM -0400, Jerome Glisse wrote:
> > > I can not take mmap_sem in range_register, the READ_ONCE is fine and
> > > they are no race as we do take a reference on the hmm struct thus
> > 
> > Of course there are use after free races with a READ_ONCE scheme, I
> > shouldn't have to explain this.
> 
> Well i can not think of anything again here the mm->hmm can not
> change while driver is calling hmm_range_register() 

Oh of course! It is so confusing because the new code makes it look like
it could be changing...

> so if you want i can remove the READ_ONCE() this does not change
> anything.

Please just write it as:

  /* The caller must hold a valid struct hmm_mirror to call this api,
   * and a valid hmm_mirror guarantees mm->hmm is valid.
   */
  range->hmm = mm->hmm;
  kref_get(&range->hmm->kref);

All the READ_ONCE/kref_get_not_zero/!hmm just obfuscates the reality
that hmm is non-NULL and constant here or the caller is using the API
wrong.

kref_get will reliably oops or WARN_ON if it is misused which is a
fine amount of debugging.

Jason