Re: [Intel-gfx] [PATCH] drm/i915: Allow dead vm to unbind vma's without lock.

[Intel]

On 1/28/22 09:57, Maarten Lankhorst wrote:

i915_gem_vm_close may take the lock, and we currently have no better way
of handling this. At least for now, allow a path in which holding vm->mutex
is sufficient. This is the case, because the object destroy path will
forcefully take vm->mutex now.

Signed-off-by: Maarten Lankhorst <maarten.lankho...@linux.intel.com>

Reviewed-by: Thomas Hellstrom <thomas.hellst...@linux.intel.com>

---
  drivers/gpu/drm/i915/i915_vma.c | 15 +++++++++++++--
  1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index b959e904c4d3..14a301c4069f 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -40,6 +40,17 @@
  #include "i915_vma.h"
  #include "i915_vma_resource.h"

+static inline void assert_vma_held_evict(const struct i915_vma *vma)

+{
+       /*
+        * We may be forced to unbind when the vm is dead, to clean it up.
+        * This is the only exception to the requirement of the object lock
+        * being held.
+        */
+       if (atomic_read(&vma->vm->open))
+               assert_object_held_shared(vma->obj);
+}
+
  static struct kmem_cache *slab_vmas;

static struct i915_vma *i915_vma_alloc(void)

@@ -1779,7 +1790,7 @@ struct dma_fence *__i915_vma_evict(struct i915_vma *vma, 
bool async)
        struct dma_fence *unbind_fence;

GEM_BUG_ON(i915_vma_is_pinned(vma));

-       assert_object_held_shared(vma->obj);
+       assert_vma_held_evict(vma);

if (i915_vma_is_map_and_fenceable(vma)) {

                /* Force a pagefault for domain tracking on next user access */
@@ -1846,7 +1857,7 @@ int __i915_vma_unbind(struct i915_vma *vma)
        int ret;

lockdep_assert_held(&vma->vm->mutex);

-       assert_object_held_shared(vma->obj);
+       assert_vma_held_evict(vma);

if (!drm_mm_node_allocated(&vma->node))

                return 0;