[clangd] Trace time the operations wait on Semaphore.

The Semaphore is currently used to limit the number of concurrently
running tasks. Tracing the wait times will allow to find out how much
time is wasted waiting on other operations to complete.

llvm-svn: 334495

clang-tools-extra/clangd/Threading.cpp

@@ -1,4 +1,5 @@
 #include "Threading.h"
+#include "Trace.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Threading.h"
@@ -23,9 +24,14 @@ void Notification::wait() const {
 Semaphore::Semaphore(std::size_t MaxLocks) : FreeSlots(MaxLocks) {}
 
 void Semaphore::lock() {
-  std::unique_lock<std::mutex> Lock(Mutex);
-  SlotsChanged.wait(Lock, [&]() { return FreeSlots > 0; });
-  --FreeSlots;
+  trace::Span Span("WaitForFreeSemaphoreSlot");
+  // trace::Span can also acquire locks in ctor and dtor, we make sure it
+  // happens when Semaphore's own lock is not held.
+  {
+    std::unique_lock<std::mutex> Lock(Mutex);
+    SlotsChanged.wait(Lock, [&]() { return FreeSlots > 0; });
+    --FreeSlots;
+  }
 }
 
 void Semaphore::unlock() {