QTlsBackendOpenSSL: Use a function-static variable in ensureLibraryLoaded()

Replace a combination of a mutex and a state variable by
a function-local variable initialized by lambda.

C++17 standard guarantees that the lambda is called only once and
that any other callers will waiting for initialization to complete.

The mutex that was replaced is also used in ensureCiphersAndCertsLoaded()
but that seems to be a false sharing.

Task-number: QTBUG-103559
Change-Id: Idb269a24b53cf3812ca9630ab4fc87f99ab16d55
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

2 files changed, 6 insertions, 10 deletions

diff --git a/src/plugins/tls/openssl/qtlsbackend_openssl.cpp b/src/plugins/tls/openssl/qtlsbackend_openssl.cpp
index ea31086fad..b454a20f2a 100644
--- a/src/plugins/tls/openssl/qtlsbackend_openssl.cpp
+++ b/src/plugins/tls/openssl/qtlsbackend_openssl.cpp
@@ -59,7 +59,6 @@ static void q_loadCiphersForConnection(SSL *connection, QList<QSslCipher> &ciphe
     }
 }
 
-bool QTlsBackendOpenSSL::s_libraryLoaded = false;
 bool QTlsBackendOpenSSL::s_loadedCiphersAndCerts = false;
 int QTlsBackendOpenSSL::s_indexForSSLExtraData = -1;
 
@@ -92,12 +91,10 @@ void QTlsBackendOpenSSL::clearErrorQueue()
 
 bool QTlsBackendOpenSSL::ensureLibraryLoaded()
 {
-    if (!q_resolveOpenSslSymbols())
-        return false;
-
-    const QMutexLocker locker(qt_opensslInitMutex());
+    static bool libraryLoaded = []() {
+        if (!q_resolveOpenSslSymbols())
+            return false;
 
-    if (!s_libraryLoaded) {
         // Initialize OpenSSL.
         if (q_OPENSSL_init_ssl(0, nullptr) != 1)
             return false;
@@ -119,10 +116,10 @@ bool QTlsBackendOpenSSL::ensureLibraryLoaded()
             return false;
         }
 
-        s_libraryLoaded = true;
-    }
+        return true;
+    }();
 
-    return true;
+    return libraryLoaded;
 }
 
 QString QTlsBackendOpenSSL::backendName() const
diff --git a/src/plugins/tls/openssl/qtlsbackend_openssl_p.h b/src/plugins/tls/openssl/qtlsbackend_openssl_p.h
index 1c5e68e0c6..748ef13ac4 100644
--- a/src/plugins/tls/openssl/qtlsbackend_openssl_p.h
+++ b/src/plugins/tls/openssl/qtlsbackend_openssl_p.h
@@ -43,7 +43,6 @@ public:
 
     static bool ensureLibraryLoaded();
     // Index used in SSL_get_ex_data to get the matching TlsCryptographerOpenSSL:
-    static bool s_libraryLoaded;
     static bool s_loadedCiphersAndCerts;
     static int s_indexForSSLExtraData;