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Fix ubsan failure in snapshot refresh (#5257) 

Summary:
The newly added test CompactionJobTest.SnapshotRefresh sets the snapshot refresh period to 0 to stress the feature. This results into large number of refresh events, which in turn results into an UBSAN failure when a bitwise shift operand goes beyond the uint64_t size.
The patch fixes that by simplifying the shift logic to be done only by 2 bits after each refresh. Furthermore it verifies that the shift operation does not result in decreasing the refresh period.

Testing:
COMPILE_WITH_UBSAN=1 make -j32 compaction_job_test
./compaction_job_test --gtest_filter=CompactionJobTest.SnapshotRefresh
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5257

Differential Revision: D15106463

Pulled By: maysamyabandeh

fbshipit-source-id: f2718898ea7ba4fa9f7e87b70cf98fe647c0de80
This commit is contained in:
Maysam Yabandeh 2019-04-26 17:26:29 -07:00 committed by Facebook Github Bot
parent 506e8448be
commit 8c7eb59838
2 changed files with 14 additions and 10 deletions
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3
db/compaction_iterator.cc
View File

@ -281,9 +281,8 @@ void CompactionIterator::NextFromInput() {
num_keys_++;
// Use num_keys_ to reduce the overhead of reading current time
if (snap_list_callback_ && snapshots_->size() &&
snap_list_callback_->TimeToRefresh(num_keys_, snap_refresh_cnt_)) {
snap_list_callback_->TimeToRefresh(num_keys_)) {
snap_list_callback_->Refresh(snapshots_, latest_snapshot_);
snap_refresh_cnt_++;
ProcessSnapshotList();
}
// First occurrence of this user key

21
db/compaction_iterator.h
View File

@ -38,15 +38,22 @@ class SnapshotListFetchCallback {
// max is the upper bound. Note: this function will acquire the db_mutex_.
virtual void Refresh(std::vector<SequenceNumber>* snapshots,
SequenceNumber max) = 0;
inline bool TimeToRefresh(size_t key_index, size_t snap_refresh_cnt) {
inline bool TimeToRefresh(const size_t key_index) {
// skip the key if key_index % every_nth_key (which is of power 2) is not 0.
if ((key_index & every_nth_key_minus_one_) != 0) {
return false;
}
// inc next refresh period exponentially (by x4)
auto next_refresh_threshold = snap_refresh_nanos_ << (snap_refresh_cnt * 2);
const uint64_t elapsed = timer_.ElapsedNanos();
return elapsed > next_refresh_threshold;
auto ret = elapsed > snap_refresh_nanos_;
// pre-compute the next time threshold
if (ret) {
// inc next refresh period exponentially (by x4)
auto next_refresh_threshold = snap_refresh_nanos_ << 2;
// make sure the shift has not overflown the highest 1 bit
snap_refresh_nanos_ =
std::max(snap_refresh_nanos_, next_refresh_threshold);
}
return ret;
}
static constexpr SnapshotListFetchCallback* kDisabled = nullptr;
@ -55,8 +62,8 @@ class SnapshotListFetchCallback {
private:
// Time since the callback was created
StopWatchNano timer_;
// The initial delay before calling ::Refresh
const uint64_t snap_refresh_nanos_;
// The delay before calling ::Refresh. To be increased exponentially.
uint64_t snap_refresh_nanos_;
// Skip evey nth key. Number n if of power 2. The math will require n-1.
const uint64_t every_nth_key_minus_one_;
};
@ -266,8 +273,6 @@ class CompactionIterator {
SnapshotListFetchCallback* snap_list_callback_;
// number of distinct keys processed
size_t num_keys_ = 0;
// number of times that snapshot list is refreshed
size_t snap_refresh_cnt_ = 0;
bool IsShuttingDown() {
// This is a best-effort facility, so memory_order_relaxed is sufficient.