On 10/4/23 17:29, Thomas Hellström wrote:
On Wed, 2023-10-04 at 14:57 +0200, Danilo Krummrich wrote:
On 10/3/23 11:11, Thomas Hellström wrote:
<snip>
+
+/**
+ * drm_gpuvm_bo_evict() - add / remove a &drm_gpuvm_bo to /
from the &drm_gpuvms
+ * evicted list
+ * @vm_bo: the &drm_gpuvm_bo to add or remove
+ * @evict: indicates whether the object is evicted
+ *
+ * Adds a &drm_gpuvm_bo to or removes it from the &drm_gpuvms
evicted list.
+ */
+void
+drm_gpuvm_bo_evict(struct drm_gpuvm_bo *vm_bo, bool evict)
+{
+ struct drm_gem_object *obj = vm_bo->obj;
+
+ dma_resv_assert_held(obj->resv);
+
+ /* Always lock list transactions, even if
DRM_GPUVM_RESV_PROTECTED is
+ * set. This is required to protect multiple concurrent
calls to
+ * drm_gpuvm_bo_evict() with BOs with different dma_resv.
+ */
This doesn't work. The RESV_PROTECTED case requires the evicted
flag we discussed before. The list is either protected by the
spinlock or the resv. Otherwise a list add could race with a list
removal elsewhere.
I think it does unless I miss something, but it might be a bit subtle
though.
Concurrent drm_gpuvm_bo_evict() are protected by the spinlock.
Additionally, when
drm_gpuvm_bo_evict() is called we hold the dma-resv of the
corresponding GEM object.
In drm_gpuvm_validate() I assert that we hold *all* dma-resv, which
implies that no
one can call drm_gpuvm_bo_evict() on any of the VM's objects and no
one can add a new
one and directly call drm_gpuvm_bo_evict() on it either.
But translated into how the data (the list in this case) is protected
it becomes
"Either the spinlock and the bo resv of a single list item OR the bo
resvs of all bos that can potentially be on the list",
while this is certainly possible to assert, any new / future code that
manipulates the evict list will probably get this wrong and as a result
the code becomes pretty fragile. I think drm_gpuvm_bo_destroy() already
gets it wrong in that it, while holding a single resv, doesn't take the
spinlock.
That's true and I don't like it either. Unfortunately, with the dma-resv
locking scheme we can't really protect the evict list without the
drm_gpuvm_bo::evicted trick properly.
But as pointed out in my other reply, I'm a bit worried about the
drm_gpuvm_bo::evicted trick being too restrictive, but maybe it's fine
doing it in the RESV_PROTECTED case.
So I think that needs fixing, and if keeping that protection I think it
needs to be documented with the list member and ideally an assert. But
also note that lockdep_assert_held will typically give false true for
dma_resv locks; as long as the first dma_resv lock locked in a drm_exec
sequence remains locked, lockdep thinks *all* dma_resv locks are held.
(or something along those lines), so the resv lockdep asserts are
currently pretty useless.
/Thomas
Thanks,
Thomas
