From: Jerome Glisse <j.glisse@gmail.com>
To: Mark Hairgrove <mhairgrove@nvidia.com>
Cc: "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"Linus Torvalds" <torvalds@linux-foundation.org>,
"joro@8bytes.org" <joro@8bytes.org>,
"Mel Gorman" <mgorman@suse.de>, "H. Peter Anvin" <hpa@zytor.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"Johannes Weiner" <jweiner@redhat.com>,
"Larry Woodman" <lwoodman@redhat.com>,
"Rik van Riel" <riel@redhat.com>,
"Dave Airlie" <airlied@redhat.com>,
"Brendan Conoboy" <blc@redhat.com>,
"Joe Donohue" <jdonohue@redhat.com>,
"Duncan Poole" <dpoole@nvidia.com>,
"Sherry Cheung" <SCheung@nvidia.com>,
"Subhash Gutti" <sgutti@nvidia.com>,
"John Hubbard" <jhubbard@nvidia.com>,
"Lucien Dunning" <ldunning@nvidia.com>,
"Cameron Buschardt" <cabuschardt@nvidia.com>,
"Arvind Gopalakrishnan" <arvindg@nvidia.com>,
"Haggai Eran" <haggaie@mellanox.com>,
"Shachar Raindel" <raindel@mellanox.com>,
"Liran Liss" <liranl@mellanox.com>,
"Roland Dreier" <roland@purestorage.com>,
"Ben Sander" <ben.sander@amd.com>,
"Greg Stoner" <Greg.Stoner@amd.com>,
"John Bridgman" <John.Bridgman@amd.com>,
"Michael Mantor" <Michael.Mantor@amd.com>,
"Paul Blinzer" <Paul.Blinzer@amd.com>,
"Laurent Morichetti" <Laurent.Morichetti@amd.com>,
"Alexander Deucher" <Alexander.Deucher@amd.com>,
"Oded Gabbay" <Oded.Gabbay@amd.com>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Jatin Kumar" <jakumar@nvidia.com>,
"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>
Subject: Re: [PATCH 05/36] HMM: introduce heterogeneous memory management v3.
Date: Mon, 15 Jun 2015 10:32:16 -0400 [thread overview]
Message-ID: <20150615143215.GA1947@gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.00.1506111520350.25907@mdh-linux64-2.nvidia.com>
On Thu, Jun 11, 2015 at 03:26:46PM -0700, Mark Hairgrove wrote:
> On Thu, 11 Jun 2015, Jerome Glisse wrote:
> > On Wed, Jun 10, 2015 at 06:15:08PM -0700, Mark Hairgrove wrote:
[...]
> > Ok i see the race you are afraid of and really it is an unlikely one
> > __mutex_unlock_common_slowpath() take a spinlock right after allowing
> > other to take the mutex, when we are in your scenario there is no
> > contention on that spinlock so it is taken right away and as there
> > is no one in the mutex wait list then it goes directly to unlock the
> > spinlock and return. You can ignore the debug function as if debugging
> > is enabled than the mutex_lock() would need to also take the spinlock
> > and thus you would have proper synchronization btw 2 thread thanks to
> > the mutex.wait_lock.
> >
> > So basicly while CPU1 is going :
> > spin_lock(mutex.wait_lock)
> > if (!list_empty(mutex.wait_list)) {
> > // wait_list is empty so branch not taken
> > }
> > spin_unlock(mutex.wait_lock)
> >
> > CPU2 would have to test the mirror list and mutex_unlock and return
> > before the spin_unlock() of CPU1. This is a tight race, i can add a
> > synchronize_rcu() to device_unregister after the mutex_unlock() so
> > that we also add a grace period before the device is potentialy freed
> > which should make that race completely unlikely.
> >
> > Moreover for something really bad to happen it would need that the
> > freed memory to be reallocated right away by some other thread. Which
> > really sound unlikely unless CPU1 is the slowest of all :)
> >
> > Cheers,
> > Jerome
> >
>
> But CPU1 could get preempted between the atomic_set and the
> spin_lock_mutex, and then it doesn't matter whether or not a grace period
> has elapsed before CPU2 proceeds.
>
> Making race conditions less likely just makes them harder to pinpoint when
> they inevitably appear in the wild. I don't think it makes sense to spend
> any effort in making a race condition less likely, and that thread I
> referenced (https://lkml.org/lkml/2013/12/2/997) is fairly strong evidence
> that fixing this race actually matters. So, I think this race condition
> really needs to be fixed.
>
> One fix is for hmm_mirror_unregister to wait for hmm_notifier_release
> completion between hmm_mirror_kill and hmm_mirror_unref. It can do this by
> calling synchronize_srcu() on the mmu_notifier's srcu. This has the
> benefit that the driver is guaranteed not to get the "mm is dead" callback
> after hmm_mirror_unregister returns.
>
> In fact, are there any callbacks on the mirror that can arrive after
> hmm_mirror_unregister? If so, how will hmm_device_unregister solve them?
>
> From a general standpoint, hmm_device_unregister must perform some kind of
> synchronization to be sure that all mirrors are completely released and
> done and no new callbacks will trigger. Since that has to be true, can't
> that synchronization be moved into hmm_mirror_unregister instead?
>
> If that happens there's no need for a "mirror can be freed" ->release
> callback at all because the driver is guaranteed that a mirror is done
> after hmm_mirror_unregister.
Well there is no need or 2 callback (relase|stop , free) just one, the
release|stop that is needed. I kind of went halfway last week on this.
I will probably rework that a little to keep just one call and rely on
driver to call hmm_mirror_unregister()
Cheers,
Jerome
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next prev parent reply other threads:[~2015-06-15 14:32 UTC|newest]
Thread overview: 79+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-21 19:31 HMM (Heterogeneous Memory Management) v8 j.glisse
2015-05-21 19:31 ` [PATCH 01/36] mmu_notifier: add event information to address invalidation v7 j.glisse
2015-05-30 3:43 ` John Hubbard
2015-06-01 19:03 ` Jerome Glisse
2015-06-01 23:10 ` John Hubbard
2015-06-03 16:07 ` Jerome Glisse
2015-06-03 23:02 ` John Hubbard
2015-05-21 19:31 ` [PATCH 02/36] mmu_notifier: keep track of active invalidation ranges v3 j.glisse
2015-05-27 5:09 ` Aneesh Kumar K.V
2015-05-27 14:32 ` Jerome Glisse
2015-06-02 9:32 ` John Hubbard
2015-06-03 17:15 ` Jerome Glisse
2015-06-05 3:29 ` John Hubbard
2015-05-21 19:31 ` [PATCH 03/36] mmu_notifier: pass page pointer to mmu_notifier_invalidate_page() j.glisse
2015-05-27 5:17 ` Aneesh Kumar K.V
2015-05-27 14:33 ` Jerome Glisse
2015-06-03 4:25 ` John Hubbard
2015-05-21 19:31 ` [PATCH 04/36] mmu_notifier: allow range invalidation to exclude a specific mmu_notifier j.glisse
2015-05-21 19:31 ` [PATCH 05/36] HMM: introduce heterogeneous memory management v3 j.glisse
2015-05-27 5:50 ` Aneesh Kumar K.V
2015-05-27 14:38 ` Jerome Glisse
2015-06-08 19:40 ` Mark Hairgrove
2015-06-08 21:17 ` Jerome Glisse
2015-06-09 1:54 ` Mark Hairgrove
2015-06-09 15:56 ` Jerome Glisse
2015-06-10 3:33 ` Mark Hairgrove
2015-06-10 15:42 ` Jerome Glisse
2015-06-11 1:15 ` Mark Hairgrove
2015-06-11 14:23 ` Jerome Glisse
2015-06-11 22:26 ` Mark Hairgrove
2015-06-15 14:32 ` Jerome Glisse [this message]
2015-05-21 19:31 ` [PATCH 06/36] HMM: add HMM page table v2 j.glisse
2015-06-19 2:06 ` Mark Hairgrove
2015-06-19 18:07 ` Jerome Glisse
2015-06-20 2:34 ` Mark Hairgrove
2015-06-25 22:57 ` Mark Hairgrove
2015-06-26 16:30 ` Jerome Glisse
2015-06-27 1:34 ` Mark Hairgrove
2015-06-29 14:43 ` Jerome Glisse
2015-07-01 2:51 ` Mark Hairgrove
2015-07-01 15:07 ` Jerome Glisse
2015-05-21 19:31 ` [PATCH 07/36] HMM: add per mirror page table v3 j.glisse
2015-06-25 23:05 ` Mark Hairgrove
2015-06-26 16:43 ` Jerome Glisse
2015-06-27 3:02 ` Mark Hairgrove
2015-06-29 14:50 ` Jerome Glisse
2015-05-21 19:31 ` [PATCH 08/36] HMM: add device page fault support v3 j.glisse
2015-05-21 19:31 ` [PATCH 09/36] HMM: add mm page table iterator helpers j.glisse
2015-05-21 19:31 ` [PATCH 10/36] HMM: use CPU page table during invalidation j.glisse
2015-05-21 19:31 ` [PATCH 11/36] HMM: add discard range helper (to clear and free resources for a range) j.glisse
2015-05-21 19:31 ` [PATCH 12/36] HMM: add dirty range helper (to toggle dirty bit inside mirror page table) j.glisse
2015-05-21 19:31 ` [PATCH 13/36] HMM: DMA map memory on behalf of device driver j.glisse
2015-05-21 19:31 ` [PATCH 14/36] fork: pass the dst vma to copy_page_range() and its sub-functions j.glisse
2015-05-21 19:31 ` [PATCH 15/36] memcg: export get_mem_cgroup_from_mm() j.glisse
2015-05-21 19:31 ` [PATCH 16/36] HMM: add special swap filetype for memory migrated to HMM device memory j.glisse
2015-06-24 7:49 ` Haggai Eran
2015-05-21 19:31 ` [PATCH 17/36] HMM: add new HMM page table flag (valid device memory) j.glisse
2015-05-21 19:31 ` [PATCH 18/36] HMM: add new HMM page table flag (select flag) j.glisse
2015-05-21 19:31 ` [PATCH 19/36] HMM: handle HMM device page table entry on mirror page table fault and update j.glisse
2015-05-21 20:22 ` [PATCH 20/36] HMM: mm add helper to update page table when migrating memory back jglisse
2015-05-21 20:22 ` [PATCH 21/36] HMM: mm add helper to update page table when migrating memory jglisse
2015-05-21 20:22 ` [PATCH 22/36] HMM: add new callback for copying memory from and to device memory jglisse
2015-05-21 20:22 ` [PATCH 23/36] HMM: allow to get pointer to spinlock protecting a directory jglisse
2015-05-21 20:23 ` [PATCH 24/36] HMM: split DMA mapping function in two jglisse
2015-05-21 20:23 ` [PATCH 25/36] HMM: add helpers for migration back to system memory jglisse
2015-05-21 20:23 ` [PATCH 26/36] HMM: fork copy migrated memory into system memory for child process jglisse
2015-05-21 20:23 ` [PATCH 27/36] HMM: CPU page fault on migrated memory jglisse
2015-05-21 20:23 ` [PATCH 28/36] HMM: add mirror fault support for system to device memory migration jglisse
2015-05-21 20:23 ` [PATCH 29/36] IB/mlx5: add a new paramter to __mlx_ib_populated_pas for ODP with HMM jglisse
2015-05-21 20:23 ` [PATCH 30/36] IB/mlx5: add a new paramter to mlx5_ib_update_mtt() " jglisse
2015-05-21 20:23 ` [PATCH 31/36] IB/odp: export rbt_ib_umem_for_each_in_range() jglisse
2015-05-21 20:23 ` [PATCH 32/36] IB/odp/hmm: add new kernel option to use HMM for ODP jglisse
2015-05-21 20:23 ` [PATCH 33/36] IB/odp/hmm: add core infiniband structure and helper for ODP with HMM jglisse
2015-06-24 13:59 ` Haggai Eran
2015-05-21 20:23 ` [PATCH 34/36] IB/mlx5/hmm: add mlx5 HMM device initialization and callback jglisse
2015-05-21 20:23 ` [PATCH 35/36] IB/mlx5/hmm: add page fault support for ODP on HMM jglisse
2015-05-21 20:23 ` [PATCH 36/36] IB/mlx5/hmm: enable ODP using HMM jglisse
2015-05-30 3:01 ` HMM (Heterogeneous Memory Management) v8 John Hubbard
2015-05-31 6:56 ` Haggai Eran
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