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Clarify buffer reclamation condition

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This commit is contained in:
Andrew Noyes
2024-05-04 18:17:44 -07:00
parent 08583f2ac0
commit 5a8275ffc1
1 changed files with 6 additions and 3 deletions
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9
RootSet.cpp
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@@ -105,12 +105,15 @@ struct RootSet::Impl {
void setOldestVersion(int64_t oldestVersion) { void setOldestVersion(int64_t oldestVersion) {
this->oldestVersion = oldestVersion; this->oldestVersion = oldestVersion;
// This condition is basically "while it would be incorrect to read // The next buffer has an entry greater than all entries in this buffer.
// firstToFree at oldestVersion - 1" // If the next buffer does not have an entry > `oldestVersion`, then this
// buffer is missing an entry <= oldestVersion on the right, so it's
// incorrect to read this buffer at `oldestVersion` and we can safely free
// it.
while (firstToFree != nullptr && firstToFree->next != nullptr && while (firstToFree != nullptr && firstToFree->next != nullptr &&
firstToFree->next->versions()[firstToFree->next->end.load( firstToFree->next->versions()[firstToFree->next->end.load(
std::memory_order_relaxed) - std::memory_order_relaxed) -
1] < oldestVersion) { 1] <= oldestVersion) {
auto *tmp = firstToFree; auto *tmp = firstToFree;
firstToFree = firstToFree->next; firstToFree = firstToFree->next;
free(tmp); free(tmp);