libroot: Add adaptive mutex implementation

headers/private/shared/locks.h

@@ -18,6 +18,7 @@ typedef struct mutex {
 } mutex;
 
 #define MUTEX_FLAG_CLONE_NAME		0x1
+#define MUTEX_FLAG_ADAPTIVE			0x2
 #define MUTEX_INITIALIZER(name)		{ name, 0, 0 }
 
 void		mutex_init(mutex *lock, const char *name);

src/system/libroot/libroot_init.c

@@ -32,6 +32,7 @@ int __libc_argc;
 char **__libc_argv;
 
 int __gABIVersion;
+int32 __gCPUCount;
 
 char _single_threaded = true;
 	// determines if I/O locking needed; needed for BeOS compatibility
@@ -48,6 +49,7 @@ int _data_offset_main_;
 void
 initialize_before(image_id imageID)
 {
+	system_info info;
 	char *programPath = __gRuntimeLoader->program_args->args[0];
 	__gCommPageAddress = __gRuntimeLoader->commpage_address;
 	__gABIVersion = __gRuntimeLoader->abi_version;
@@ -70,6 +72,9 @@ initialize_before(image_id imageID)
 
 	pthread_self()->id = find_thread(NULL);
 
+	get_system_info(&info);
+	__gCPUCount = info.cpu_count;
+
 	__init_time((addr_t)__gCommPageAddress);
 	__init_heap();
 	__init_env(__gRuntimeLoader->program_args);

src/system/libroot/os/locks/mutex.cpp

@@ -18,6 +18,12 @@
 #include <user_mutex_defs.h>
 
 
+#define MAX_UNSUCCESSFUL_SPINS	100
+
+
+extern int32 __gCPUCount;
+
+
 // #pragma mark - mutex
 
 
@@ -36,6 +42,9 @@ mutex_init_etc(mutex *lock, const char *name, uint32 flags)
 	lock->name = (flags & MUTEX_FLAG_CLONE_NAME) != 0 ? strdup(name) : name;
 	lock->lock = 0;
 	lock->flags = flags;
+
+	if (__gCPUCount < 2)
+		lock->flags &= ~uint32(MUTEX_FLAG_ADAPTIVE);
 }
 
 
@@ -50,15 +59,22 @@ mutex_destroy(mutex *lock)
 status_t
 mutex_lock(mutex *lock)
 {
-	// set the locked flag
-	int32 oldValue = atomic_or(&lock->lock, B_USER_MUTEX_LOCKED);
+	uint32 count;
+	const uint32 kMaxCount
+		= (lock->flags & MUTEX_FLAG_ADAPTIVE) != 0 ? MAX_UNSUCCESSFUL_SPINS : 1;
 
-	if ((oldValue & (B_USER_MUTEX_LOCKED | B_USER_MUTEX_WAITING)) == 0
-			|| (oldValue & B_USER_MUTEX_DISABLED) != 0) {
-		// No one has the lock or is waiting for it, or the mutex has been
-		// disabled.
-		return B_OK;
-	}
+	int32 oldValue;
+	do {
+		// set the locked flag
+		oldValue = atomic_or(&lock->lock, B_USER_MUTEX_LOCKED);
+
+		if ((oldValue & (B_USER_MUTEX_LOCKED | B_USER_MUTEX_WAITING)) == 0
+				|| (oldValue & B_USER_MUTEX_DISABLED) != 0) {
+			// No one has the lock or is waiting for it, or the mutex has been
+			// disabled.
+			return B_OK;
+		}
+	} while (count++ < kMaxCount && (oldValue & B_USER_MUTEX_WAITING) != 0);
 
 	// we have to call the kernel
 	status_t error;

src/system/libroot/posix/malloc/arch-specific.cpp

@@ -362,7 +362,7 @@ hoardUnsbrk(void *ptr, long size)
 void
 hoardLockInit(hoardLockType &lock, const char *name)
 {
-	mutex_init(&lock, name);
+	mutex_init_etc(&lock, name, MUTEX_FLAG_ADAPTIVE);
 }
 
 

src/system/runtime_loader/runtime_loader.cpp

@@ -24,6 +24,8 @@
 struct user_space_program_args *gProgramArgs;
 void *__gCommPageAddress;
 
+int32 __gCPUCount = 1;
+
 
 static const char *
 search_path_for_type(image_type type)