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Skiplist - GC - race condition fix

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Summary: An unpleasant race-condition detected. Solution proposed. It's not very pretty. Perhaps consider using the ConcurrentPushQueue. Not 100% sure, but it should make the GC code easier to work with.

Reviewers: buda, mislav.bradac, teon.banek, dgleich

Reviewed By: buda

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D469
This commit is contained in:
florijan 2017-06-14 13:15:48 +02:00
parent 928f52af47
commit b6da65b9e7
1 changed files with 10 additions and 3 deletions
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13
src/data_structures/concurrent/skiplist_gc.hpp
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@ -110,12 +110,19 @@ class SkipListGC : public Loggable {
// We can only modify this in a not-thread safe way because we are the only
// thread ever accessing it here, i.e. there is at most one thread doing
// this GarbageCollection.
// find the oldest not deletable record
// since we can't copy a concurrent list iterator, we must use two
// separate ones, while ensuring they point to the same record
// in the beginning of the search-loop
auto it = deleted_list_.begin();
auto oldest_not_deletable = deleted_list_.begin();
while (oldest_not_deletable != it) oldest_not_deletable++;
// we need a bool to track if oldest_not_deletable should get
// deleted too (we have not skipped any record due to safe_id condition)
bool delete_all = true;
for (auto it = deleted_list_.begin(); it != deleted_list_.end(); ++it) {
for (; it != deleted_list_.end(); ++it) {
if (it->first > safe_id) {
// We have to increase iterator manually because the copy assignment
// operator is deleted.
while (oldest_not_deletable != it) ++oldest_not_deletable;
delete_all = false;
}