From 4e3fdfc2d6482d42b8e152911e24ee38b1bafc89 Mon Sep 17 00:00:00 2001
From: Alexander Gololobov <davenger@clickhouse.com>
Date: Fri, 19 Jul 2024 13:26:35 +0200
Subject: [PATCH 1/3] Save writer thread id for debugging

---
 src/Common/SharedMutex.cpp | 10 +++++++++-
 src/Common/SharedMutex.h   |  2 ++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/Common/SharedMutex.cpp b/src/Common/SharedMutex.cpp
index 1df09ca998a..7b00ef0b28b 100644
--- a/src/Common/SharedMutex.cpp
+++ b/src/Common/SharedMutex.cpp
@@ -1,4 +1,5 @@
 #include <Common/SharedMutex.h>
+#include <base/getThreadId.h>
 
 #ifdef OS_LINUX /// Because of futex
 
@@ -12,6 +13,7 @@ namespace DB
 SharedMutex::SharedMutex()
     : state(0)
     , waiters(0)
+    , writer_thread_id(0)
 {}
 
 void SharedMutex::lock()
@@ -32,16 +34,22 @@ void SharedMutex::lock()
     value |= writers;
     while (value & readers)
         futexWaitLowerFetch(state, value);
+
+    writer_thread_id.store(getThreadId());
 }
 
 bool SharedMutex::try_lock()
 {
     UInt64 value = 0;
-    return state.compare_exchange_strong(value, writers);
+    bool success = state.compare_exchange_strong(value, writers);
+    if (success)
+        writer_thread_id.store(getThreadId());
+    return success;
 }
 
 void SharedMutex::unlock()
 {
+    writer_thread_id.store(0);
     state.store(0);
     if (waiters)
         futexWakeUpperAll(state);
diff --git a/src/Common/SharedMutex.h b/src/Common/SharedMutex.h
index 9215ff62af3..a53e2984239 100644
--- a/src/Common/SharedMutex.h
+++ b/src/Common/SharedMutex.h
@@ -36,6 +36,8 @@ private:
 
     alignas(64) std::atomic<UInt64> state;
     std::atomic<UInt32> waiters;
+    /// Is set while the lock is held in exclusive mode only to facilitate debugging
+    std::atomic<UInt64> writer_thread_id;
 };
 
 }

From 55d1656f4d0da2f23b2df719dabeed7999645349 Mon Sep 17 00:00:00 2001
From: Alexander Gololobov <davenger@clickhouse.com>
Date: Fri, 19 Jul 2024 13:27:41 +0200
Subject: [PATCH 2/3] Moving is not safe, prohibit it

---
 src/Common/SharedMutex.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/Common/SharedMutex.h b/src/Common/SharedMutex.h
index a53e2984239..c77c8765885 100644
--- a/src/Common/SharedMutex.h
+++ b/src/Common/SharedMutex.h
@@ -19,6 +19,8 @@ public:
     ~SharedMutex() = default;
     SharedMutex(const SharedMutex &) = delete;
     SharedMutex & operator=(const SharedMutex &) = delete;
+    SharedMutex(SharedMutex &&) = delete;
+    SharedMutex & operator=(SharedMutex &&) = delete;
 
     // Exclusive ownership
     void lock() TSA_ACQUIRE();

From bb0b29f6e50e74098fcc8a9b83150998f1bc2601 Mon Sep 17 00:00:00 2001
From: Alexander Gololobov <davenger@clickhouse.com>
Date: Mon, 22 Jul 2024 12:11:56 +0200
Subject: [PATCH 3/3] Set writer_thread_id earlier, when new exclusive owener
 is waiting for existing readers to finish

---
 src/Common/SharedMutex.cpp | 6 ++++--
 src/Common/SharedMutex.h   | 2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/Common/SharedMutex.cpp b/src/Common/SharedMutex.cpp
index 7b00ef0b28b..2d63f8542b0 100644
--- a/src/Common/SharedMutex.cpp
+++ b/src/Common/SharedMutex.cpp
@@ -31,11 +31,13 @@ void SharedMutex::lock()
             break;
     }
 
+    /// The first step of acquiring the exclusive ownership is finished.
+    /// Now we just wait until all readers release the shared ownership.
+    writer_thread_id.store(getThreadId());
+
     value |= writers;
     while (value & readers)
         futexWaitLowerFetch(state, value);
-
-    writer_thread_id.store(getThreadId());
 }
 
 bool SharedMutex::try_lock()
diff --git a/src/Common/SharedMutex.h b/src/Common/SharedMutex.h
index c77c8765885..d2947645eca 100644
--- a/src/Common/SharedMutex.h
+++ b/src/Common/SharedMutex.h
@@ -38,7 +38,7 @@ private:
 
     alignas(64) std::atomic<UInt64> state;
     std::atomic<UInt32> waiters;
-    /// Is set while the lock is held in exclusive mode only to facilitate debugging
+    /// Is set while the lock is held (or is in the process of being acquired) in exclusive mode only to facilitate debugging
     std::atomic<UInt64> writer_thread_id;
 };