From a0deec960f3a8190831c673e5ba998fe6fb7ea90 Mon Sep 17 00:00:00 2001
From: Changli Gao <xiaosuo@gmail.com>
Date: Sun, 20 Nov 2016 18:14:33 -0800
Subject: [PATCH] Fix deadlock when calling getMergedHistogram

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
---
 util/statistics.cc | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/util/statistics.cc b/util/statistics.cc
index a27001eea..051956839 100644
--- a/util/statistics.cc
+++ b/util/statistics.cc
@@ -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);