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From: Jerome Glisse <jglisse@redhat.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Evgeny Baskakov <ebaskakov@nvidia.com>,
	Ralph Campbell <rcampbell@nvidia.com>,
	Mark Hairgrove <mhairgrove@nvidia.com>
Subject: Re: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2
Date: Wed, 21 Mar 2018 19:41:10 -0400	[thread overview]
Message-ID: <20180321234110.GK3214@redhat.com> (raw)
In-Reply-To: <a9ba54c5-a2d9-49f6-16ad-46b79525b93c@nvidia.com>

On Wed, Mar 21, 2018 at 04:22:49PM -0700, John Hubbard wrote:
> On 03/21/2018 11:16 AM, jglisse@redhat.com wrote:
> > From: Jérôme Glisse <jglisse@redhat.com>
> > 
> > This code was lost in translation at one point. This properly call
> > mmu_notifier_unregister_no_release() once last user is gone. This
> > fix the zombie mm_struct as without this patch we do not drop the
> > refcount we have on it.
> > 
> > Changed since v1:
> >   - close race window between a last mirror unregistering and a new
> >     mirror registering, which could have lead to use after free()
> >     kind of bug
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> > Cc: Evgeny Baskakov <ebaskakov@nvidia.com>
> > Cc: Ralph Campbell <rcampbell@nvidia.com>
> > Cc: Mark Hairgrove <mhairgrove@nvidia.com>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > ---
> >  mm/hmm.c | 35 +++++++++++++++++++++++++++++++++--
> >  1 file changed, 33 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 6088fa6ed137..f75aa8df6e97 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -222,13 +222,24 @@ int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
> >  	if (!mm || !mirror || !mirror->ops)
> >  		return -EINVAL;
> >  
> > +again:
> >  	mirror->hmm = hmm_register(mm);
> >  	if (!mirror->hmm)
> >  		return -ENOMEM;
> >  
> >  	down_write(&mirror->hmm->mirrors_sem);
> > -	list_add(&mirror->list, &mirror->hmm->mirrors);
> > -	up_write(&mirror->hmm->mirrors_sem);
> > +	if (mirror->hmm->mm == NULL) {
> > +		/*
> > +		 * A racing hmm_mirror_unregister() is about to destroy the hmm
> > +		 * struct. Try again to allocate a new one.
> > +		 */
> > +		up_write(&mirror->hmm->mirrors_sem);
> > +		mirror->hmm = NULL;
> 
> This is being set outside of locks, so now there is another race with
> another hmm_mirror_register...
> 
> I'll take a moment and draft up what I have in mind here, which is a more
> symmetrical locking scheme for these routines.
> 

No this code is correct. hmm->mm is set after hmm struct is allocated
and before it is public so no one can race with that. It is clear in
hmm_mirror_unregister() under the write lock hence checking it here
under that same lock is correct.

Cheers,
Jérôme

  reply	other threads:[~2018-03-21 23:41 UTC|newest]

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-20  2:00 [PATCH 00/15] hmm: fixes and documentations v3 jglisse
2018-03-20  2:00 ` [PATCH 01/15] mm/hmm: documentation editorial update to HMM documentation jglisse
2018-03-20  2:00 ` [PATCH 02/15] mm/hmm: fix header file if/else/endif maze v2 jglisse
2018-03-20  2:00 ` [PATCH 03/15] mm/hmm: HMM should have a callback before MM is destroyed v2 jglisse
2018-03-21  4:14   ` John Hubbard
2018-03-21 18:03     ` Jerome Glisse
2018-03-21 22:16       ` John Hubbard
2018-03-21 22:46         ` Jerome Glisse
2018-03-21 23:10           ` John Hubbard
2018-03-21 23:37             ` Jerome Glisse
2018-03-22  0:11               ` John Hubbard
2018-03-22  1:32                 ` Jerome Glisse
2018-03-22  1:28   ` [PATCH 03/15] mm/hmm: HMM should have a callback before MM is destroyed v3 jglisse
2018-03-22  6:58     ` John Hubbard
2018-03-20  2:00 ` [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit jglisse
2018-03-21  4:24   ` John Hubbard
2018-03-21 18:12     ` Jerome Glisse
2018-03-21 18:16   ` [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2 jglisse
2018-03-21 23:22     ` John Hubbard
2018-03-21 23:41       ` Jerome Glisse [this message]
2018-03-22 22:47         ` John Hubbard
2018-03-22 23:37           ` Jerome Glisse
2018-03-23  0:13             ` John Hubbard
2018-03-23  0:50               ` Jerome Glisse
2018-03-23  0:56                 ` John Hubbard
2018-03-22  1:30     ` [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v3 jglisse
2018-03-22 22:36       ` Andrew Morton
2018-03-20  2:00 ` [PATCH 05/15] mm/hmm: hmm_pfns_bad() was accessing wrong struct jglisse
2018-03-20  2:00 ` [PATCH 06/15] mm/hmm: use struct for hmm_vma_fault(), hmm_vma_get_pfns() parameters v2 jglisse
2018-03-20  2:00 ` [PATCH 07/15] mm/hmm: remove HMM_PFN_READ flag and ignore peculiar architecture v2 jglisse
2018-03-20  2:00 ` [PATCH 08/15] mm/hmm: use uint64_t for HMM pfn instead of defining hmm_pfn_t to ulong v2 jglisse
2018-03-20  2:00 ` [PATCH 09/15] mm/hmm: cleanup special vma handling (VM_SPECIAL) jglisse
2018-03-20  2:00 ` [PATCH 10/15] mm/hmm: do not differentiate between empty entry or missing directory v2 jglisse
2018-03-21  5:24   ` John Hubbard
2018-03-21 14:48     ` Jerome Glisse
2018-03-21 23:16       ` John Hubbard
2018-03-20  2:00 ` [PATCH 11/15] mm/hmm: rename HMM_PFN_DEVICE_UNADDRESSABLE to HMM_PFN_DEVICE_PRIVATE jglisse
2018-03-20  2:00 ` [PATCH 12/15] mm/hmm: move hmm_pfns_clear() closer to where it is use jglisse
2018-03-20  2:00 ` [PATCH 13/15] mm/hmm: factor out pte and pmd handling to simplify hmm_vma_walk_pmd() jglisse
2018-03-21  5:07   ` John Hubbard
2018-03-21 15:08     ` Jerome Glisse
2018-03-21 22:36       ` John Hubbard
2018-03-20  2:00 ` [PATCH 14/15] mm/hmm: change hmm_vma_fault() to allow write fault on page basis jglisse
2018-03-20  2:00 ` [PATCH 15/15] mm/hmm: use device driver encoding for HMM pfn v2 jglisse
2018-03-21  4:39   ` John Hubbard
2018-03-21 15:52     ` Jerome Glisse
2018-03-21 23:19       ` John Hubbard

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