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Fix deadlock when calling getMergedHistogram

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Summary:
When calling StatisticsImpl::HistogramInfo::getMergedHistogram(), if
there is a dying thread, which is calling
ThreadLocalPtr::StaticMeta::OnThreadExit() to merge its thread values to
HistogramInfo, deadlock will occur. Because the former try to hold
merge_lock then ThreadMeta::mutex_, but the later try to hold
ThreadMeta::mutex_ then merge_lock. In short, the locking order isn't
the same.

This patch addressed this issue by releasing merge_lock before folding
thread values.
Closes https://github.com/facebook/rocksdb/pull/1552

Differential Revision: D4211942

Pulled By: ajkr

fbshipit-source-id: ef89bcb
This commit is contained in:
Changli Gao 2016-11-20 18:14:33 -08:00 committed by Islam AbdelRahman
parent 41526f44f6
commit 8e68ffb872
1 changed files with 4 additions and 2 deletions
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6
util/statistics.cc
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@ -51,9 +51,11 @@ uint64_t StatisticsImpl::getTickerCount(uint32_t tickerType) const {
std::unique_ptr<HistogramImpl>
StatisticsImpl::HistogramInfo::getMergedHistogram() const {
MutexLock lock(&merge_lock);
std::unique_ptr<HistogramImpl> res_hist(new HistogramImpl());
res_hist->Merge(merged_hist);
{
MutexLock lock(&merge_lock);
res_hist->Merge(merged_hist);
}
thread_value->Fold(
[](void* curr_ptr, void* res) {
auto tmp_res_hist = static_cast<HistogramImpl*>(res);