On Sun, Oct 22, 2023 at 08:02:36PM +0200, Thomas Hellström 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.
>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> ---
> Documentation/gpu/drm-vm-bind-locking.rst | 494 ++++++++++++++++++++++
> 1 file changed, 494 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..c290ff4287fb
> --- /dev/null
> +++ b/Documentation/gpu/drm-vm-bind-locking.rst
> @@ -0,0 +1,494 @@
> +.. 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.
> +* ``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. 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, the backing store of
> + which is anonymous pages as described above.
something strange after the comma, but my bad english can't allow to pin
point what. Or maybe it is right and the problem *is* my bad english :)
> +* ``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.
> +* ``dma_resv``: A struct dma_resv (AKA reservation object) that is used
maybe s/AKA/a.k.a ?!
> + 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.
> +* ``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``: AKA external object: A GEM object which may be shared
maybe s/AKA/a.k.a ?!
> + by multiple gpu_vms and whose backing storage may be shared with
> + other drivers.
> +
> +
> +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.
> +* 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.
> +
^^ spurious spaces
> +.. _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. 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.
> +
> +
> +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.
> +
^^^^^^^^^^^^^ more spurious spaces
> + 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);
> +
> +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.
> +
(pausing here... I will continue the review below tomorrow)
> +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);
> + 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 eviction blit or copy will wait for GPU idle, any 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
> +the gpu_vma is rebound. The eviction code holding the object's dma_resv while
> +revalidating will ensure a new exec function may not race with the eviction.
> +
> +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
> +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. 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.
> +
> +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);
> + 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
> +===========================================================
> +
> +Many drivers will not need to access the gpu_vm's evict- and
> +external objects lists without holding the gpu_vm's dma_resv lock,
> +but some drivers do, 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 over the lists, the items already iterated over needs
> +to be 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*
> +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
> +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,
> +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. 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.
> +
> +.. 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.
> +
> +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. 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.
> +
> --
> 2.41.0
>
