crypto: replace rwlocks with simple mutexes

It was pointed out by Zhou Ran that the Windows XP implementation of
uv_rwlock_rdlock() and friends may unlock the inner write mutex on a
different thread than the one that locked it, resulting in undefined
behavior.

The only place that uses rwlocks is the crypto module.  Make that use
normal (simple) mutexes instead.

OpenSSL's critical sections are generally very short, with exclusive
access outnumbering shared access by a factor of three or more, so
it's not as if using rwlocks gives a decisive performance advantage.

PR-URL: https://github.com/nodejs/node/pull/2723
Reviewed-By: Fedor Indutny <fedor@indutny.com>

src/node_crypto.cc

@@ -123,7 +123,7 @@ struct ClearErrorOnReturn {
   ~ClearErrorOnReturn() { ERR_clear_error(); }
 };
 
-static uv_rwlock_t* locks;
+static uv_mutex_t* locks;
 
 const char* const root_certs[] = {
 #include "node_root_certs.h"  // NOLINT(build/include_order)
@@ -178,29 +178,22 @@ static void crypto_lock_init(void) {
   int i, n;
 
   n = CRYPTO_num_locks();
-  locks = new uv_rwlock_t[n];
+  locks = new uv_mutex_t[n];
 
   for (i = 0; i < n; i++)
-    if (uv_rwlock_init(locks + i))
+    if (uv_mutex_init(locks + i))
       abort();
 }
 
 
 static void crypto_lock_cb(int mode, int n, const char* file, int line) {
-  CHECK((mode & CRYPTO_LOCK) || (mode & CRYPTO_UNLOCK));
-  CHECK((mode & CRYPTO_READ) || (mode & CRYPTO_WRITE));
+  CHECK(!(mode & CRYPTO_LOCK) ^ !(mode & CRYPTO_UNLOCK));
+  CHECK(!(mode & CRYPTO_READ) ^ !(mode & CRYPTO_WRITE));
 
-  if (mode & CRYPTO_LOCK) {
-    if (mode & CRYPTO_READ)
-      uv_rwlock_rdlock(locks + n);
-    else
-      uv_rwlock_wrlock(locks + n);
-  } else {
-    if (mode & CRYPTO_READ)
-      uv_rwlock_rdunlock(locks + n);
-    else
-      uv_rwlock_wrunlock(locks + n);
-  }
+  if (mode & CRYPTO_LOCK)
+    uv_mutex_lock(locks + n);
+  else
+    uv_mutex_unlock(locks + n);
 }