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Fix: race condition in atomic disjoint set

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To fix this, I compared our implementation with the reference implementation
in more detail again and found some differences.
Aligning our and the reference implementation a bit more fixed the issue.

Reference implementation: https://github.com/wjakob/dset/blob/master/dset.h

Pull Request: https://projects.blender.org/blender/blender/pulls/118434
This commit is contained in:
Jacques Lucke
2024-02-18 19:20:45 +01:00
parent 0e01fba353
commit 02127c1d3e
1 changed files with 14 additions and 10 deletions
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24
source/blender/blenlib/BLI_atomic_disjoint_set.hh
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@@ -55,33 +55,37 @@ class AtomicDisjointSet {
return;
}
Item x_item = items_[x].load(relaxed);
Item y_item = items_[y].load(relaxed);
int x_rank = items_[x].load(relaxed).rank;
int y_rank = items_[y].load(relaxed).rank;
if (
/* Implement union by rank heuristic. */
x_item.rank > y_item.rank
x_rank > y_rank
/* If the rank is the same, make a consistent decision. */
|| (x_item.rank == y_item.rank && x < y))
|| (x_rank == y_rank && x < y))
{
std::swap(x_item, y_item);
std::swap(x_rank, y_rank);
std::swap(x, y);
}
/* Update parent of item x. */
const Item x_item_new{y, x_item.rank};
if (!items_[x].compare_exchange_strong(x_item, x_item_new, relaxed)) {
Item x_item_old = {x, x_rank};
const Item x_item_new{y, x_rank};
if (!items_[x].compare_exchange_strong(x_item_old, x_item_new, relaxed)) {
/* Another thread has updated item x, start again. */
continue;
}
if (x_item.rank == y_item.rank) {
if (x_rank == y_rank) {
/* Increase rank of item y. This may fail when another thread has updated item y in the
* meantime. That may lead to worse behavior with the union by rank heurist, but seems to
* be ok in practice. */
const Item y_item_new{y, y_item.rank + 1};
items_[y].compare_exchange_weak(y_item, y_item_new, relaxed);
Item y_item_old{y, y_rank};
const Item y_item_new{y, y_rank + 1};
items_[y].compare_exchange_weak(y_item_old, y_item_new, relaxed);
}
return;
}
}