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: Thu, 22 Mar 2018 20:50:17 -0400 [thread overview]
Message-ID: <20180323005017.GB5011@redhat.com> (raw)
In-Reply-To: <b858d92a-3a38-bfff-fe66-697c64ea2053@nvidia.com>
On Thu, Mar 22, 2018 at 05:13:14PM -0700, John Hubbard wrote:
> On 03/22/2018 04:37 PM, Jerome Glisse wrote:
> > On Thu, Mar 22, 2018 at 03:47:16PM -0700, John Hubbard wrote:
> >> On 03/21/2018 04:41 PM, Jerome Glisse wrote:
> >>> 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: Jerome Glisse <jglisse@redhat.com>
>
> <snip>
>
> >>>
> >>> 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.
> >>
> >> Are you implying that code that calls hmm_mirror_register() should do
> >> it's own locking, to prevent simultaneous calls to that function? Because
> >> as things are right now, multiple threads can arrive at this point. The
> >> fact that mirror->hmm is not "public" is irrelevant; what matters is that
> >>> 1 thread can change it simultaneously.
> >
> > The content of struct hmm_mirror should not be modified by code outside
> > HMM after hmm_mirror_register() and before hmm_mirror_unregister(). This
> > is a private structure to HMM and the driver should not touch it, ie it
> > should be considered as read only/const from driver code point of view.
>
> Yes, that point is clear and obvious.
>
> >
> > It is also expected (which was obvious to me) that driver only call once
> > and only once hmm_mirror_register(), and only once hmm_mirror_unregister()
> > for any given hmm_mirror struct. Note that driver can register multiple
> > _different_ mirror struct to same mm or differents mm.
> >
> > There is no need of locking on the driver side whatsoever as long as the
> > above rules are respected. I am puzzle if they were not obvious :)
>
> Those rules were not obvious. It's unusual to claim that register and unregister
> can run concurrently, but regiser and register cannot. Let's please document
> the rules a bit in the comments.
I am really surprise this was not obvious. All existing _register API
in the kernel follow this. You register something once only and doing
it twice for same structure (ie unique struct hmm_mirror *mirror pointer
value) leads to serious bugs (doing so concurently or not).
For instance if you call mmu_notifier_register() twice (concurrently
or not) with same pointer value for struct mmu_notifier *mn then bad
thing will happen. Same for driver_register() but this one actualy
have sanity check and complain loudly if that happens. I doubt there
is any single *_register/unregister() in the kernel that does not
follow this.
Note that doing register/unregister concurrently for the same unique
hmm_mirror struct is also illegal. However concurrent register and
unregister of different hmm_mirror struct is legal and this is the
reasons for races we were discussing.
Cheers,
Jerome
next prev parent reply other threads:[~2018-03-23 0:50 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
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 [this message]
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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