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    SubjectRe: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2
    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: Jérôme 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,
    Jérôme

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