Bug 1312087 - part 0 - use PTHREAD_MUTEX_ADAPTIVE_NP mutexes on Linux/glibc; r=erahm

This matches NSPR's behavior for its PRLock type.

mozglue/misc/Mutex_posix.cpp

@@ -27,22 +27,35 @@ mozilla::detail::MutexImpl::MutexImpl()
 {
   pthread_mutexattr_t* attrp = nullptr;
 
-#ifdef DEBUG
+  // glibc's adaptive mutexes spin for a short number of tries before sleeping.
+  // NSPR's locks did this, too, and it seems like a reasonable thing to do.
+#if defined(__linux__) && defined(__GLIBC__)
+#define ADAPTIVE_MUTEX_SUPPORTED
+#endif
+
+#if defined(DEBUG)
+#define ATTR_REQUIRED
+#define MUTEX_KIND PTHREAD_MUTEX_ERRORCHECK
+#elif defined(ADAPTIVE_MUTEX_SUPPORTED)
+#define ATTR_REQUIRED
+#define MUTEX_KIND PTHREAD_MUTEX_ADAPTIVE_NP
+#endif
+
+#if defined(ATTR_REQUIRED)
   pthread_mutexattr_t attr;
 
   TRY_CALL_PTHREADS(pthread_mutexattr_init(&attr),
                     "mozilla::detail::MutexImpl::MutexImpl: pthread_mutexattr_init failed");
 
-  TRY_CALL_PTHREADS(pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK),
+  TRY_CALL_PTHREADS(pthread_mutexattr_settype(&attr, MUTEX_KIND),
                     "mozilla::detail::MutexImpl::MutexImpl: pthread_mutexattr_settype failed");
-
   attrp = &attr;
 #endif
 
   TRY_CALL_PTHREADS(pthread_mutex_init(&platformData()->ptMutex, attrp),
                     "mozilla::detail::MutexImpl::MutexImpl: pthread_mutex_init failed");
 
-#ifdef DEBUG
+#if defined(ATTR_REQUIRED)
   TRY_CALL_PTHREADS(pthread_mutexattr_destroy(&attr),
                     "mozilla::detail::MutexImpl::MutexImpl: pthread_mutexattr_destroy failed");
 #endif