SERVER-127310 Remove blocking work on the connections server status section (#54357)

GitOrigin-RevId: 2bec2fd3ac7d4052b16341a4d62c89214ca1a8be

src/mongo/transport/service_executor_reserved.cpp

@@ -41,6 +41,7 @@
 #include "mongo/util/functional.h"
 #include "mongo/util/out_of_line_executor.h"
 
+#include <algorithm>
 #include <mutex>
 #include <utility>
 
@@ -84,7 +85,7 @@ void ServiceExecutorReserved::start() {
     {
         std::unique_lock<std::mutex> lk(_mutex);
         _stillRunning.store(true);
-        _numStartingThreads = _reservedThreads;
+        _numStartingThreads.store(_reservedThreads);
     }
 
     for (size_t i = 0; i < _reservedThreads; i++) {
@@ -102,8 +103,8 @@ Status ServiceExecutorReserved::_startWorker() {
             _shutdownCondition.notify_one();
         });
 
-        _numStartingThreads--;
-        _numReadyThreads++;
+        _numStartingThreads.subtractAndFetch(1);
+        _numReadyThreads.addAndFetch(1);
 
         while (_stillRunning.load()) {
             _threadWakeup.wait(lk, [&] { return (!_stillRunning.load() || !_readyTasks.empty()); });
@@ -118,10 +119,11 @@ Status ServiceExecutorReserved::_startWorker() {
 
             auto task = std::move(_readyTasks.front());
             _readyTasks.pop_front();
-            _numReadyThreads -= 1;
+            _numReadyThreads.subtractAndFetch(1);
             bool launchReplacement = false;
-            if (_numReadyThreads + _numStartingThreads < _reservedThreads) {
-                _numStartingThreads++;
+            // lk is still needed here to prevent a toctou bug.
+            if (_numReadyThreads.load() + _numStartingThreads.load() < _reservedThreads) {
+                _numStartingThreads.addAndFetch(1);
                 launchReplacement = true;
             }
 
@@ -133,9 +135,7 @@ Status ServiceExecutorReserved::_startWorker() {
                     LOGV2_WARNING(22981,
                                   "Could not start new reserve worker thread",
                                   "error"_attr = threadStartStatus);
-                    lk.lock();
-                    _numStartingThreads--;
-                    lk.unlock();
+                    _numStartingThreads.subtractAndFetch(1);
                 }
             }
 
@@ -146,10 +146,10 @@ Status ServiceExecutorReserved::_startWorker() {
             }
 
             lk.lock();
-            if (_numReadyThreads + 1 > _reservedThreads) {
+            if (_numReadyThreads.load() + 1 > _reservedThreads) {
                 break;
             } else {
-                _numReadyThreads += 1;
+                _numReadyThreads.addAndFetch(1);
             }
         }
 
@@ -208,9 +208,11 @@ void ServiceExecutorReserved::appendStats(BSONObjBuilder* bob) const {
     };
 
     auto statlet = [&] {
-        std::lock_guard lk(_mutex);
         auto threads = static_cast<int>(_numRunningWorkerThreads.loadRelaxed());
-        auto total = static_cast<int>(threads - _numReadyThreads - _numStartingThreads);
+        auto ready = static_cast<int>(_numReadyThreads.loadRelaxed());
+        auto starting = static_cast<int>(_numStartingThreads.loadRelaxed());
+        // Clamp to 0 in case there is a race condition where ready + starting > threads.
+        auto total = std::max(threads - ready - starting, 0);
         auto running = total;
         auto waiting = 0;
         return Statlet{threads, total, running, waiting};

src/mongo/transport/service_executor_reserved.h

@@ -98,8 +98,8 @@ private:
     std::deque<Task> _readyTasks;
 
     AtomicWord<unsigned> _numRunningWorkerThreads{0};
-    size_t _numReadyThreads{0};
-    size_t _numStartingThreads{0};
+    Atomic<size_t> _numReadyThreads{0};
+    Atomic<size_t> _numStartingThreads{0};
 
     const std::string _name;
     const size_t _reservedThreads;

src/mongo/transport/session_manager_common.cpp

@@ -434,8 +434,7 @@ bool SessionManagerCommon::waitForNoSessions(Milliseconds timeout) {
 }
 
 std::size_t SessionManagerCommon::numOpenSessions() const {
-    auto sync = _sessions->sync();
-    return sync.size();
+    return _sessions->size();
 }
 
 std::vector<std::pair<SessionId, std::string>> SessionManagerCommon::getOpenSessionIDs() const {