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Fix race condition between log_.erase and log_.back

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Summary:
log_ contract specifies that it should not be modified unless both mutex_ and log_write_mutex_ are held. log_.erase however does that with only holding mutex_. This causes a race condition with two_write_queues since logs_.back is read with holding only log_write_mutex_ (which is correct according to logs_ contract) but logs_.erase is called concurrently. This is probably the cause of logs_.back returning nullptr in https://github.com/facebook/rocksdb/issues/3852 although I could not reproduce it.
Fixes https://github.com/facebook/rocksdb/issues/3852
Closes https://github.com/facebook/rocksdb/pull/3859

Differential Revision: D8026103

Pulled By: maysamyabandeh

fbshipit-source-id: ee394e00fe4aa520d884c5ef87981e9d6b5ccb28
This commit is contained in:
Maysam Yabandeh 2018-05-16 12:51:31 -07:00 committed by Facebook Github Bot
parent 42cb4775c1
commit 46fde6b653
1 changed files with 2 additions and 0 deletions
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2
db/db_impl.cc
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@ -810,6 +810,8 @@ void DBImpl::MarkLogsSynced(uint64_t up_to, bool synced_dir,
assert(log.getting_synced);
if (status.ok() && logs_.size() > 1) {
logs_to_free_.push_back(log.ReleaseWriter());
// To modify logs_ both mutex_ and log_write_mutex_ must be held
InstrumentedMutexLock l(&log_write_mutex_);
it = logs_.erase(it);
} else {
log.getting_synced = false;