Re: [PATCH v4] Documentation/gpu: VM_BIND locking document

Boris Brezillon
2023-11-16T09:49:04Z

Hi Thomas,

On Wed, 15 Nov 2023 13:49:37 +0100
Thomas Hellström <thomas.hellstrom@linux.intel.com> wrote:

> Add the first version of the VM_BIND locking document which is
> intended to be part of the xe driver upstreaming agreement.
>
> The document describes and discuss the locking used during exec-
> functions, evicton and for userptr gpu-vmas. Intention is to be using the
> same nomenclature as the drm-vm-bind-async.rst.
>
> v2:
> - s/gvm/gpu_vm/g (Rodrigo Vivi)
> - Clarify the userptr seqlock with a pointer to mm/mmu_notifier.c
> (Rodrigo Vivi)
> - Adjust commit message accordingly.
> - Add SPDX license header.
>
> v3:
> - Large update to align with the drm_gpuvm manager locking
> - Add "Efficient userptr gpu_vma exec function iteration" section
> - Add "Locking at bind- and unbind time" section.
>
> v4:
> - Fix tabs vs space errors by untabifying (Rodrigo Vivi)
> - Minor style fixes and typos (Rodrigo Vivi)
> - Clarify situations where stale GPU mappings are occurring and how
> access through these mappings are blocked. (Rodrigo Vivi)
> - Insert into the toctree in implementation_guidelines.rst
>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
> Documentation/gpu/drm-vm-bind-locking.rst | 503 ++++++++++++++++++
> .../gpu/implementation_guidelines.rst | 1 +
> 2 files changed, 504 insertions(+)
> create mode 100644 Documentation/gpu/drm-vm-bind-locking.rst
>
> diff --git a/Documentation/gpu/drm-vm-bind-locking.rst b/Documentation/gpu/drm-vm-bind-locking.rst
> new file mode 100644
> index 000000000000..bc701157cb34
> --- /dev/null
> +++ b/Documentation/gpu/drm-vm-bind-locking.rst
> @@ -0,0 +1,503 @@
> +.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
> +
> +===============
> +VM_BIND locking
> +===============
> +
> +This document attempts to describe what's needed to get VM_BIND locking right,
> +including the userptr mmu_notifier locking and it will also discuss some
> +optimizations to get rid of the looping through of all userptr mappings and
> +external / shared object mappings that is needed in the simplest
> +implementation. It will also discuss some implications for faulting gpu_vms.
> +
> +Nomenclature
> +============
> +
> +* ``Context``: GPU execution context.
> +* ``gpu_vm``: Abstraction of a virtual GPU address space with
> + meta-data. Typically one per client (DRM file-private), or one per
> + context.

Should we mention that it's a driver object, likely inheriting from
drm_gpuvm?

> +* ``gpu_vma``: Abstraction of a GPU address range within a gpu_vm with
> + associated meta-data. The backing storage of a gpu_vma can either be
> + a GEM object or anonymous pages mapped also into the CPU
> + address space for the process.
> +* gpu_vm_bo: Abstracts the association of a GEM object and
> + a VM. Note that if only one gpu_vma per vm and buffer object were
> + allowed, the state stored with a gpu_vm_bo could just as well have
> + been stored with the gpu_vma.

I find this note confusing, and I don't think it brings any value to
the rest of the explanation. Sure, there's a way we could have
optimized things for 1:1 VM:BO mappings, but do we care about this very
restrictive use case in this context.

> For the purpose of this document, each
> + GEM object maintains a list of gpu_vm_bos, and each gpu_vm_bo
> + maintains a list of gpu_vmas.
> +* ``userptr gpu_vma or just userptr``: A gpu_vma, whose backing store
> + is anonymous pages as described above.
> +* ``revalidating``: Revalidating a gpu_vma means making the latest version
> + of the backing store resident and making sure the gpu_vma's
> + page-table entries point to that backing store.
> +* ``dma_fence``: A struct dma_fence that is similar to a struct completion
> + and which tracks GPU activity. When the GPU activity is finished,
> + the dma_fence signals.

Maybe we could point to the dma_fence doc here [1]

> +* ``dma_resv``: A struct dma_resv (a.k.a reservation object) that is used
> + to track GPU activity in the form of multiple dma_fences on a
> + gpu_vm or a GEM object. The dma_resv contains an array / list
> + of dma_fences and a lock that needs to be held when adding
> + additional dma_fences to the dma_resv. The lock is of a type that
> + allows deadlock-safe locking of multiple dma_resvs in arbitrary order.

Same for dma_resv [2]

> +* ``exec function``: An exec function is a function that revalidates all
> + affected gpu_vmas, submits a GPU command batch and registers the
> + dma_fence representing the GPU command's activity with all affected
> + dma_resvs. For completeness, although not covered by this document,
> + it's worth mentioning that an exec function may also be the
> + revalidation worker that is used by some drivers in compute /
> + long-running mode.
> +* ``local object``: A GEM object which is local to a gpu_vm. Shared gem
> + objects also share the gpu_vm's dma_resv.
> +* ``shared object``: a.k.a external object: A GEM object which may be shared
> + by multiple gpu_vms and whose backing storage may be shared with
> + other drivers.

Should we name that one external object and mentions it's sometimes
also called external object. This way it matches the name used in gpuvm
implementation (extobj).

> +
> +
> +Locks used and locking orders
> +=============================
> +
> +One of the benefits of VM_BIND is that local GEM objects share the gpu_vm's
> +dma_resv object and hence the dma_resv lock. So even with a huge
> +number of local GEM objects, only one lock is needed to make the exec
> +sequence atomic.
> +
> +The following locks and locking orders are used:
> +
> +* The ``gpu_vm->lock`` (optionally an rwsem). Protects how the gpu_vm is
> + partitioned into gpu_vmas. It can also protect the gpu_vm's list of
> + userptr gpu_vmas. With a CPU mm analogy this would correspond to the
> + mmap_lock.

I don't see any drm_gpuvm::lock field in Danilo's latest patchset, so,
unless I missed one version, and this lock is actually provided by
drm_gpuvm, I would mention this is a driver-specific lock. This comment
applies to all the locks you describe here actually (mention which ones
are provided by drm_gpuvm, and which ones are driver-specific).

> +* The ``userptr_seqlock``. This lock is taken in read mode for each
> + userptr gpu_vma on the gpu_vm's userptr list, and in write mode during mmu
> + notifier invalidation. This is not a real seqlock but described in
> + ``mm/mmu_notifier.c`` as a "Collision-retry read-side/write-side
> + 'lock' a lot like a seqcount, however this allows multiple
> + write-sides to hold it at once...". The read side critical section
> + is enclosed by ``mmu_interval_read_begin() /
> + mmu_interval_read_retry()`` with ``mmu_interval_read_begin()``
> + sleeping if the write side is held.
> + The write side is held by the core mm while calling mmu interval
> + invalidation notifiers.
> +* The ``gpu_vm->resv`` lock. Protects the gpu_vm's list of gpu_vmas needing
> + rebinding, and also the residency of all the gpu_vm's local GEM object.
> + Furthermore it typically protects the gpu_vm's list of evicted GEM
> + objects and external objects.
> +* The ``gpu_vm->userptr_notifier_lock``. This is an rwsem that is
> + taken in read mode during exec and write mode during a mmu notifier
> + invalidation. The userptr notifier lock is per gpu_vm.
> +* The gpu_vm list spinlocks. With some implementations they are needed
> + to be able to update the gpu_vm evicted- and external object
> + list. For those implementations, the spinlocks are grabbed when the
> + lists are manipulated. However to avoid locking order violations
> + with the dma_resv locks, a special scheme is needed when iterating
> + over the lists.
> +
> +.. _gpu_vma lifetime:
> +
> +Protection and lifetime of gpu_vm_bos and gpu_vmas
> +==================================================
> +
> +The GEM object's list of gpu_vm_bos is typically protected by the
> +GEM object's dma_resv.

Should we mention the driver-custom lock here, for cases where the
driver needs to link a gpu_vm_bo to the GEM list in a dma-signalling
path?

> Each gpu_vm_bo holds a reference counted pointer
> +to the underlying GEM object, and each gpu_vma holds a reference counted
> +pointer to the gpu_vm_bo. When iterating over the GEM object's
> +list of gpu_vm_bos the gem object's dma_resv must thus be held,
> +but if it needs to be dropped during the iteration, care needs to be
> +taken so that any gpu_vm_bo, and the gpu_vm, if dereferenced
> +while the lock is dropped, do not disappear. The easiest way to avoid
> +this is to take a reference on affected objects while the dma_resv is
> +still held. If iterating over the gpu_vm_bo's gpu_vmas, even
> +greater care needs to be taken since the gpu_vmas are not
> +reference counted. If a driver accesses a gpu_vma obtained from
> +the gpu_vm_bo's list of gpu_vmas, and the GEM object's
> +dma_resv is dropped, at the very least, it should be thoroughly
> +documented how the gpu_vma is kept alive. Otherwise holding the
> +GEM object's dma_resv lock also around unlinking a gpu_vma from a
> +gpu_vm_bo will ensure that doesn't happen.

If we have use cases where accessing VMAs without the GEM object
gpuva_list lock held is useful, it would probably be worth mentioning
them. If not, I'm not sure we should list it as a possibility, given
how hard it'd be to guarantee that the VMA is still valid when being
accessed in that situation (I mean, that could be done with some
driver-specific refcounting at the VMA object level, but is it really
something we want to encourage drivers to do?).

> +
> +
> +Revalidation and eviction of local objects
> +==========================================
> +
> +Revalidation
> +____________
> +With VM_BIND, all local objects need to be resident when the gpu is
> +executing using the gpu_vm, and the objects need to have valid
> +gpu_vmas set up pointing to them. Typically each gpu command buffer
> +submission is therefore preceded with a re-validation section:
> +
> +.. code-block:: C
> +
> + dma_resv_lock(gpu_vm->resv);
> +
> + // Validation section starts here.
> + for_each_gpu_vm_bo_on_evict_list(&gpu_vm->evict_list, &gpu_vm_bo) {
> + validate_gem_bo(&gpu_vm_bo->gem_bo);
> +
> + // The following list iteration needs the Gem object's
> + // dma_resv to be held (it protects the gpu_vm_bo's list of
> + // gpu_vmas, but since local gem objects share the gpu_vm's
> + // dma_resv, it is already held at this point.
> + for_each_gpu_vma_of_gpu_vm_bo(&gpu_vm_bo, &gpu_vma)
> + move_gpu_vma_to_rebind_list(&gpu_vma, &gpu_vm->rebind_list);
> + }
> +
> + for_each_gpu_vma_on_rebind_list(&gpu vm->rebind_list, &gpu_vma) {
> + rebind_gpu_vma(&gpu_vma);
> + remove_gpu_vma_from_rebind_list(&gpu_vma);
> + }
> + // Validation section ends here, and job submission starts.
> +
> + add_dependencies(&gpu_job, &gpu_vm->resv);
> + job_dma_fence = gpu_submit(&gpu_job));
> +
> + add_dma_fence(job_dma_fence, &gpu_vm->resv);
> + dma_resv_unlock(gpu_vm->resv);

This code excerpt is lacking all the retryable resv-locking + slot
reservation steps. I mean, we'd normally use a drm_exec (or
drm_gpuvm_exec) context to acquire the VM resvs, all extobjs resvs +
all other driver internal resvs that are not attached to any of the VM
BOs. I wonder if this simplification is not going to cause trouble in
the long run, but at the very least, it should be mentioned that this
is a simplified version.

> +
> +The reason for having a separate gpu_vm rebind list is that there
> +might be userptr gpu_vmas that are not mapping a buffer object that
> +also need rebinding.
> +
> +Eviction
> +________
> +
> +Eviction of one of these local objects will then look similar to the
> +following:
> +
> +.. code-block:: C
> +
> + obj = get_object_from_lru();
> +
> + dma_resv_lock(obj->resv);
> + for_each_gpu_vm_bo_of_obj(obj, &gpu_vm_bo);
> + add_gpu_vm_bo_to_evict_list(&gpu_vm_bo, &gpu_vm->evict_list);
> +
> + add_dependencies(&eviction_job, &obj->resv);
> + job_dma_fence = gpu_submit(&eviction_job);

Could you expend a bit on what the eviction job does? On embedded GPUs,
where there's no VRAM, we typically don't have a GPU job to teardown
GPU mappings. We do have an asynchronous VM_BIND queue though, so I
suspect the job here is an async VM_BIND job.

Not asking you to add that to the doc, I'm just trying to figure out how
this would map to the mem-reclaim logic in panthor...

> + add_dma_fence(&obj->resv, job_dma_fence);
> +
> + dma_resv_unlock(&obj->resv);
> + put_object(obj);
> +
> +Note that since the object is local to the gpu_vm, it will share the gpu_vm's
> +dma_resv lock so that ``obj->resv == gpu_vm->resv``.
> +The gpu_vm_bos marked for eviction are put on the gpu_vm's evict list,
> +which is protected by ``gpu_vm->resv``, that is always locked while
> +evicting, due to the above equality.
> +
> +For VM_BIND gpu_vms, gpu_vmas don't need to be unbound before eviction,
> +Since the driver must ensure that the eviction blit or copy will wait
> +for GPU idle or depend on all previous GPU activity. Furthermore, any
> +subsequent attempt by the GPU to access freed memory through the
> +gpu_vma will be preceded by a new exec function, with a revalidation
> +section which will make sure all gpu_vmas are rebound. The eviction
> +code holding the object's dma_resv while revalidating will ensure a
> +new exec function may not race with the eviction. Note that this will
> +not hold true, however, if only a subsets of vmas are, due to the
> +driver implementation, selected for rebinding the next exec
> +function.

This last sentence is hard to follow.

"
Note that this will not hold true if only a subset of vmas
are selected for rebinding during the next exec call (for instance, due
to some driver decision to only partially restore VMAs).
"

> Then all vmas *not* selected for rebinding needs to be
> +properly unbound before re-enabling GPU access to the VM.

I think I get the problem, but can we have a use case where partial
VMA restoration is useful? I mean, if some VMAs are not needed, we
might be able to save MMU page table allocation/setup-time, but given
the mess it then is to track those non-live VMAs, I'm wondering if it's
leaving the door open for that, unless there's a good reason to do it.

> +
> +
> +Locking with external (or shared) buffer objects
> +================================================
> +
> +Since shared buffer objects may be shared by multiple gpu_vm's they
> +can't share their reservation object with a single gpu_vm, but will rather
> +have a reservation object of their own. The shared objects bound to a
> +gpu_vm using one or many gpu_vmas are therefore typically put on a
> +per-gpu_vm list which is protected by the gpu_vm's dma_resv lock. Once
> +the gpu_vm's reservation object is locked, it is safe to traverse the

^ extra space

> +external object list and lock the dma_resvs of all external objects.
> +
> +At eviction time we now need to put the gpu_vm_bos of *all* gpu_vms a
> +shared object is bound to on the gpu_vm's evict list, but we can no longer
> +be certain that we hold the gpu_vm's dma_resv of all the gpu_vms the
> +object is bound to, since at eviction time we only hold the object's
> +private dma_resv. If we have a ww_acquire context at hand at eviction
> +time we could grab the those dma_resvs but that could cause
> +expensive ww_mutex rollbacks. A simple option is to just mark the
> +gpu_vm_bos of the evicted gem object with an ``evicted`` bool that
> +is inspected the next time the corresponding gpu_vm evicted list needs
> +to be traversed.

IIUC, the vm_bo is not added to the evicted list immediately in that
case, so I suspect you meant the next time the corresponding gpu_vm
*extobj* list needs to be traversed.

> At that time the gpu_vm's dma_resv and the object's
> +dma_resv is held, and the gpu_vm_bo marked evicted, can then be added
> +to the gpu_vm's list of evicted gpu_vm_bos. The ``evicted`` bool would
> +then be protected by the object's dma_resv.

I would be more asserted here:

"
Note that accesses to the ``evicted`` bool are protected by the
GEM object's dma_resv.
"

> +
> +The exec function would then become
> +
> +.. code-block:: C
> +
> + dma_resv_lock(gpu_vm->resv);
> +
> + // External object list is protected by the gpu_vm->resv lock.
> + for_each_gpu_vm_bo_on_extobj_list(gpu_vm, &gpu_vm_bo) {
> + dma_resv_lock(gpu_vm_bo.gem_obj->resv);

Ok, so it's really pseudo-code where you're assuming dma_resv_lock()
calls are actually locks with a ww_acquire context. This really needs
to be clarified to people don't copy/paste this code directly.

> + if (gpu_vm_bo_marked_evicted(&gpu_vm_bo))
> + add_gpu_vm_bo_to_evict_list(&gpu_vm_bo, &gpu_vm->evict_list);
> + }
> +
> + for_each_gpu_vm_bo_on_evict_list(&gpu_vm->evict_list, &gpu_vm_bo) {
> + validate_gem_bo(&gpu_vm_bo->gem_bo);
> +
> + for_each_gpu_vma_of_gpu_vm_bo(&gpu_vm_bo, &gpu_vma)
> + move_gpu_vma_to_rebind_list(&gpu_vma, &gpu_vm->rebind_list);
> + }
> +
> + for_each_gpu_vma_on_rebind_list(&gpu vm->rebind_list, &gpu_vma) {
> + rebind_gpu_vma(&gpu_vma);
> + remove_gpu_vma_from_rebind_list(&gpu_vma);
> + }
> +
> + add_dependencies(&gpu_job, &gpu_vm->resv);
> + job_dma_fence = gpu_submit(&gpu_job));
> +
> + add_dma_fence(job_dma_fence, &gpu_vm->resv);
> + for_each_shared_obj(gpu_vm, &obj)
> + add_dma_fence(job_dma_fence, &obj->resv);
> + dma_resv_unlock_all_resv_locks();
> +
> +And the corresponding shared-object aware eviction would look like:
> +
> +.. code-block:: C
> +
> + obj = get_object_from_lru();
> +
> + dma_resv_lock(obj->resv);
> + for_each_gpu_vm_bo_of_obj(obj, &gpu_vm_bo)
> + if (object_is_vm_local(obj))
> + add_gpu_vm_bo_to_evict_list(&gpu_vm_bo, &gpu_vm->evict_list);
> + else
> + mark_gpu_vm_bo_evicted(&gpu_vm_bo);
> +
> + add_dependencies(&eviction_job, &obj->resv);
> + job_dma_fence = gpu_submit(&eviction_job);
> + add_dma_fence(&obj->resv, job_dma_fence);
> +
> + dma_resv_unlock(&obj->resv);
> + put_object(obj);
> +
> +.. _Spinlock iteration:
> +
> +Accessing the gpu_vm's lists without the dma_resv lock held
> +===========================================================
> +
> +Although some drivers will hold the gpu_vm's dma_resv lock when
> +accessing the gpu_vm's evict list and external objects lists, there
> +are drivers that need to access these lists without the dma_resv lock held,
> +for example due to asynchronous state updates from within the
> +dma_fence signalling critical path. In such case a spinlock can be
> +used to protect manipulation of the lists. However, since higher level
> +sleeping locks needs to be taken for each list item while iterating

^ need

> +over the lists, the items already iterated over needs to be

^ need

> +temporarily moved to a private list and the spinlock released
> +while processing each item:
> +
> +.. code block:: C
> +
> + struct list_head still_in_list;
> +
> + INIT_LIST_HEAD(&still_in_list);
> +
> + spin_lock(&gpu_vm->list_lock);
> + do {
> + struct list_head *entry = list_first_entry_or_null(&gpu_vm->list, head);
> +
> + if (!entry)
> + break;
> +
> + list_move_tail(&entry->head, &still_in_list);
> + list_entry_get_unless_zero(entry);
> + spin_unlock(&gpu_vm->list_lock);
> +
> + process(entry);
> +
> + spin_lock(&gpu_vm->list_lock);
> + list_entry_put(entry);
> + } while (true);
> +
> + list_splice_tail(&still_in_list, &gpu_vm->list);
> + spin_unlock(&gpu_vm->list_lock);
> +
> +However, due to the additional locking and atomic operations, drivers that *can*

I would drop the 'However'.

> +avoid accessing the gpu_vm's list outside of the dma_resv lock
> +might want to avoid this iteration scheme, if the driver anticipates a
> +large number of list items. For lists where the anticipated number of
> +list items is small, list iteration doesn't happen very often, or
> +there is a significant additional cost associated with each iteration,
> +the atomic operation overhead associated with this type of iteration
> +is, however, probably negligible. Note that if this scheme is

'however' can be dropped here as well.

> +used, it is necessary to make sure this list iteration is protected by
> +an outer level lock or semaphore, since list items are temporarily
> +pulled off the list while iterating.
> +
> +TODO: Pointer to the gpuvm code implementation if this iteration and
> +how to choose either iteration scheme.
> +
> +userptr gpu_vmas
> +================
> +
> +A userptr gpu_vma is a gpu_vma that, instead of mapping a buffer object to a
> +GPU virtual address range, directly maps a CPU mm range of anonymous-
> +or file page-cache pages.
> +A very simple approach would be to just pin the pages using
> +pin_user_pages() at bind time and unpin them at unbind time, but this
> +creates a Denial-Of-Service vector since a single user-space process
> +would be able to pin down all of system memory, which is not
> +desirable. (For special use-cases and with proper accounting pinning might
> +still be a desirable feature, though). What we need to do in the
> +general case is to obtain a reference to the desired pages, make sure
> +we are notified using a MMU notifier just before the CPU mm unmaps the
> +pages, dirty them if they are not mapped read-only to the GPU, and
> +then drop the reference.
> +When we are notified by the MMU notifier that CPU mm is about to drop the
> +pages, we need to stop GPU access to the pages by waiting for VM idle
> +in the MMU notifier and make sure that before the next time the GPU
> +tries to access whatever is now present in the CPU mm range, we unmap
> +the old pages from the GPU page tables and repeat the process of
> +obtaining new page references. (See the :ref:`notifier example
> +<Invalidation example>` below). Note that when the core mm decides to
> +laundry pages, we get such an unmap MMU notification and can mark the
> +pages dirty again before the next GPU access. We also get similar MMU
> +notifications for NUMA accounting which the GPU driver doesn't really
> +need to care about, but so far it has proven difficult to exclude
> +certain notifications.
> +
> +Using a MMU notifier for device DMA (and other methods) is described in
> +`this document
> +<https://docs.kernel.org/core-api/pin_user_pages.html#case-3-mmu-notifier-registration-with-or-without-page-faulting-hardware>`_.
> +
> +Now the method of obtaining struct page references using
> +get_user_pages() unfortunately can't be used under a dma_resv lock
> +since that would violate the locking order of the dma_resv lock vs the
> +mmap_lock that is grabbed when resolving a CPU pagefault. This means
> +the gpu_vm's list of userptr gpu_vmas needs to be protected by an
> +outer lock.
> +
> +The MMU interval seqlock for a userptr gpu_vma is used in the following
> +way:
> +
> +.. code-block:: C
> +
> + // Exclusive locking mode here is strictly needed only if there are
> + // invalidated userptr vmas present, to avoid multiple userptr
> + // revalidations.
> + down_write(&gpu_vm->lock);
> + retry:
> +
> + // Note: mmu_interval_read_begin() blocks until there is no
> + // invalidation notifier running anymore.
> + seq = mmu_interval_read_begin(&gpu_vma->userptr_interval);
> + if (seq != gpu_vma->saved_seq) {
> + obtain_new_page_pointers(&gpu_vma);
> + dma_resv_lock(&gpu_vm->resv);
> + add_gpu_vma_top_revalidate_list(&gpu_vma, &gpu_vm);
> + dma_resv_unlock(&gpu_vm->resv);
> + gpu_vma->saved_seq = seq;
> + }
> +
> + // The usual revalidation goes here.
> +
> + // Final userptr sequence validation may not happen before the
> + // submission dma_fence is added to the gpu_vm's resv, from the POW
> + // of the MMU invalidation notifier. Hence the
> + // userptr_notifier_lock that will make them appear atomic.
> +
> + add_dependencies(&gpu_job, &gpu_vm->resv);
> + down_read(&gpu_vm->userptr_notifier_lock);
> + if (mmu_interval_read_retry(&gpu_vma->userptr_interval, gpu_vma->saved_seq)) {
> + up_read(&gpu_vm->userptr_notifier_lock);
> + goto retry;
> + }
> +
> + job_dma_fence = gpu_submit(&gpu_job));
> +
> + add_dma_fence(job_dma_fence, &gpu_vm->resv);
> +
> + for_each_shared_obj(gpu_vm, &obj)
> + add_dma_fence(job_dma_fence, &obj->resv);
> +
> + dma_resv_unlock_all_resv_locks();
> + up_read(&gpu_vm->userptr_notifier_lock);
> + up_write(&gpu_vm->lock);
> +
> +The code between ``mmu_interval_read_begin()`` and the
> +``mmu_interval_read_retry()`` marks the read side critical section of
> +what we call the ``userptr_seqlock``. In reality the gpu_vm's userptr
> +gpu_vma list is looped through, and the check is done for *all* of its
> +userptr gpu_vmas, although we only show a single one here.
> +
> +The userptr gpu_vma MMU invalidation notifier might be called from
> +reclaim context and, again to avoid locking order violations, we can't
> +take any dma_resv lock nor the gpu_vm->lock from within it.
> +
> +.. _Invalidation example:
> +.. code-block:: C
> +
> + bool gpu_vma_userptr_invalidate(userptr_interval, cur_seq)
> + {
> + // Make sure the exec function either sees the new sequence
> + // and backs off or we wait for the dma-fence:
> +
> + down_write(&gpu_vm->userptr_notifier_lock);
> + mmu_interval_set_seq(userptr_interval, cur_seq);
> + up_write(&gpu_vm->userptr_notifier_lock);
> +
> + // At this point, the exec function can't succeed in
> + // submitting a new job, because cur_seq is an invalid
> + // sequence number and will always cause a retry. When all
> + // invalidation callbacks, the mmu notifier core will flip
> + // the sequence number to a valid one. However we need to
> + // stop gpu access to the old pages here.
> +
> + dma_resv_wait_timeout(&gpu_vm->resv, DMA_RESV_USAGE_BOOKKEEP,
> + false, MAX_SCHEDULE_TIMEOUT);
> + return true;
> + }
> +
> +When this invalidation notifier returns, the GPU can no longer be
> +accessing the old pages of the userptr gpu_vma and needs to redo the
> +page-binding before a new GPU submission can succeed.

Don't know enough about userptr to properly review this section, sorry
:-/.

> +
> +Efficient userptr gpu_vma exec_function iteration
> +_________________________________________________
> +
> +If the gpu_vm's list of userptr gpu_vmas becomes large, it's
> +inefficient to iterate through the complete lists of userptrs on each
> +exec function to check whether each userptr gpu_vma's saved
> +sequence number is invalid or stale. A solution to this is to put all
> +*invalidated* userptr gpu_vmas on a separate gpu_vm list and
> +only those gpu_vmas on the list are actually checked on each exec
> +function. This list will then lend itself very-well to the spinlock
> +locking scheme that is
> +:ref:`described in the spinlock iteration section <Spinlock iteration>`, since
> +in the mmu notifier, where we add the invalidated gpu_vmas to the
> +list, it's not possible to take any outer locks like the
> +``gpu_vm->lock`` or the ``gpu_vm->resv`` lock. Note that the
> +``gpu_vm->lock`` still needs to be taken while iterating to ensure the list is
> +complete, as also mentioned in that section.
> +
> +If using an invalidated userptr list like this, the retry check in the
> +exec function trivially becomes a check for invalidated list empty.
> +
> +Locking at bind and unbind time
> +================================
> +
> +At bind time, assuming a GEM object backed gpu_vma, each
> +gpu_vma needs to be associated with a gpu_vm_bo and that
> +gpu_vm_bo in turn needs to be added to the GEM object's
> +gpu_vm_bo list, and possibly to the gpu_vm's external object
> +list. This is referred to as *linking* the gpu_vma, and typically
> +requires that the ``gpu_vm->resv`` and the GEM object's dma_resv are
> +held.

Again, depends on the locking scheme you picked. While I agree that
insertion/removal of extobjs is likely to be done with the VM and GEM
resv held, link/unlink being done when the VMA is
inserted/removed in the tree, it really depends on the VM update model
chosen by the driver (update early/at-VM_BIND-ioctl-time vs
JIT/at-VM_BIND-job-exec-time).

We went over it quite a few times with Danilo and you, and I keep
forgetting why early link/deferred unlink (leaving the VMA tree update
in the dma-signalling path, but the link/unlink being done outside of
this path) wouldn't work. I think it'd be worth documenting this kind of
details, so we don't come to discuss it over and over again, just to
end with the exact same conclusion :-). Maybe we should add a "drm_gpuvm
design decisions" section gathering all those tricky/hard to grasp
details at some point.

> When unlinking a gpu_vma the same locks are typically held,
> +and that ensures, as briefly discussed
> +:ref:`previously <gpu_vma lifetime>`, that when iterating over
> +``gpu_vmas`, either under the ``gpu_vm->resv`` or the GEM
> +object's dma_resv, that the gpu_vmas stay alive as long
> +as the lock under which we iterate are not is not released. For
> +userptr gpu_vmas it's similarly required that during unlink, the
> +outer ``gpu_vm->lock`` is held, since otherwise when iterating over
> +the invalidated userptr list as described in the previous section,
> +there is nothing keeping those userptr gpu_vmas alive.
> diff --git a/Documentation/gpu/implementation_guidelines.rst b/Documentation/gpu/implementation_guidelines.rst
> index 138e637dcc6b..dbccfa72f1c9 100644
> --- a/Documentation/gpu/implementation_guidelines.rst
> +++ b/Documentation/gpu/implementation_guidelines.rst
> @@ -7,3 +7,4 @@ Misc DRM driver uAPI- and feature implementation guidelines
> .. toctree::
>
> drm-vm-bind-async
> + drm-vm-bind-locking

Looks pretty good overall. My only complaint would be that you don't
refer much to drm_gpuvm, when it's actually suppose to become the
common ground for GPU VM handling in drivers (at least that was my
understanding).

Regards,

Boris

[1]https://docs.kernel.org/driver-api/dma-buf.html#dma-fences
[2]https://docs.kernel.org/driver-api/dma-buf.html#reservation-objects