Fix #109583: Avoid allocation new custom loop normals for write #109671

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Iliya Katushenock wants to merge 3 commits from mod_moder:tmp_fix_data_race_to_compute_normals into blender-v3.6-release

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Curve Deform node has never worked correctly.
This used to be a leaked copy made by each node
to write its own version of the custom loop normals.
Const correctness and sharing increased the chances of a crash.

Curve Deform node has never worked correctly. This used to be a leaked copy made by each node to write its own version of the custom loop normals. Const correctness and sharing increased the chances of a crash.
Iliya Katushenock added the
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labels 2023-07-03 22:20:37 +02:00
Iliya Katushenock requested review from Jacques Lucke 2023-07-03 22:20:55 +02:00
Iliya Katushenock requested review from Hans Goudey 2023-07-03 22:20:55 +02:00
Iliya Katushenock added this to the Nodes & Physics project 2023-07-03 22:20:59 +02:00
Iliya Katushenock force-pushed tmp_fix_data_race_to_compute_normals from e42064db77 to a61d126618 2023-07-03 22:29:35 +02:00 Compare
Iliya Katushenock changed title from Fix #109583: Const version of mesh_calc_normals to only eval normals in geometry nodes to Fix #109583: Const version of mesh_calc_normals to only eval normals in geometry nodes 2023-07-03 22:29:39 +02:00
mod_moder changed target branch from main to blender-v3.6-release 2023-07-03 22:29:50 +02:00
Iliya Katushenock added 1 commit 2023-07-03 22:30:56 +02:00
Iliya Katushenock added 1 commit 2023-07-04 01:24:19 +02:00
Iliya Katushenock changed title from Fix #109583: Const version of mesh_calc_normals to only eval normals in geometry nodes to Fix #109583: Avoid allocation new custom loop normals for write 2023-07-04 02:07:17 +02:00
Jacques Lucke reviewed 2023-07-04 10:13:24 +02:00
@ -1890,0 +1887,4 @@
/* May be nullptr. Const_cast to avoid data race while normals computing from different threads
* for the same mesh. */
blender::short2 *clnors = const_cast<blender::short2 *>(static_cast<const blender::short2 *>(
CustomData_get_layer(&mesh->ldata, CD_CUSTOMLOOPNORMAL)));
Member

That does not seem quite right to me. How can BKE_mesh_calc_normals_split_ex know that it is allowed to use const_cast and ignore implicit sharing?

That does not seem quite right to me. How can `BKE_mesh_calc_normals_split_ex` know that it is allowed to use `const_cast` and ignore implicit sharing?
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The problem was exactly that 10 nodes were taking it for the writing. Since there is only one real owner (only the object itself), different threads create new owners (that is, they simply add a reference count) and copy the data. But the data itself must continue to belong to the object. That is, now, the object must own all copies of this data for different threads. I really don't know how to fix this...
This doesn't look right.
For this reason, I'd rather just have a different version of the function (which would ignore this data). But Hans' suggestion was that it doesn't work just because of const correctness. If the streams will only read, everything should be fine. But they can still make a changes...
And a future normal refactoring might fix that..

The problem was exactly that 10 nodes were taking it for the writing. Since there is only one real owner (only the object itself), different threads create new owners (that is, they simply add a reference count) and copy the data. But the data itself must continue to belong to the object. That is, now, the object must own all copies of this data for different threads. I really don't know how to fix this... This doesn't look right. For this reason, I'd rather just have a different version of the function (which would ignore this data). But Hans' suggestion was that it doesn't work just because of const correctness. If the streams will only read, everything should be fine. But they can still make a changes... And a future normal refactoring might fix that..
Member

If this is just a cache, then it should be protected by a mutex. I probably don't fully understand what's going on here right now.

If this is just a cache, then it should be protected by a mutex. I probably don't fully understand what's going on here right now.
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10 curve deform nodes are asked to compute normals on a single object. Each of the nodes, through this function, requires the creation of local data for writing. And either a leak occurs, or now the reference counter is causing a crash ...

10 curve deform nodes are asked to compute normals on a single object. Each of the nodes, through this function, requires the creation of local data for writing. And either a leak occurs, or now the reference counter is causing a crash ...
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Member

Although, now it seems to me that if each node creates its own copy of the mesh, implicit sharing will work correctly here.

Although, now it seems to me that if each node creates its own copy of the mesh, implicit sharing will work correctly here.
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https://projects.blender.org/blender/blender/pulls/109695
Iliya Katushenock changed title from Fix #109583: Avoid allocation new custom loop normals for write to Fix #109583: Avoid allocation new custom loop normals for write 2023-07-04 15:11:41 +02:00
Iliya Katushenock changed title from Fix #109583: Avoid allocation new custom loop normals for write to Fix #109583: Avoid allocation new custom loop normals for write 2023-07-04 15:15:57 +02:00

Pull request closed

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Reference: blender/blender#109671
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