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From:   =?UTF-8?q?Thomas=20Hellstr=C3=B6m?= 
        <thomas.hellstrom@linux.intel.com>
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        <thomas.hellstrom@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Matthew Brost <matthew.brost@intel.com>,
        Danilo Krummrich <dakr@redhat.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Oak Zeng <oak.zeng@intel.com>, Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Francois Dugast <francois.dugast@intel.com>,
        Boris Brezillon <boris.brezillon@collabora.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: [PATCH v3] Documentation/gpu: VM_BIND locking document
Date:   Sun, 22 Oct 2023 20:02:36 +0200
Message-ID: <20231022180236.5170-1-thomas.hellstrom@linux.intel.com>
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Precedence: bulk

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@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 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.
+* ``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
+  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
+  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.
+  
+.. _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.
+		
+   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.
+
+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