kernel: Print the first 4 chars of cvar names in KDL "threads" command.

Condition variables are now a pretty common way the kernel blocks threads.
That means the "threads" command was getting difficult to navigate, since
at any given time, a lot of threads could be blocked on "cvar".

Now we try (carefully, because it could fault!) to fetch the first 4
characters of the "type" name and display then. This suffices to
distinguish the most common object block types in the list at a glance
(e.g. "cvar:port" for port reads, the most common.)

Change-Id: I94f4b59fd78b7ebdce913944551a5e98f0ca2e33
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6605
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

headers/private/kernel/condition_variable.h

@@ -97,6 +97,8 @@ protected:
 
 			friend struct ConditionVariableEntry;
 			friend struct ConditionVariableHashDefinition;
+			friend ssize_t debug_condition_variable_type_strlcpy(ConditionVariable* cvar,
+				char* name, size_t size);
 };
 
 

src/system/kernel/condition_variable.cpp

@@ -47,46 +47,6 @@ static ConditionVariableHash sConditionVariableHash;
 static rw_spinlock sConditionVariableHashLock;
 
 
-static int
-list_condition_variables(int argc, char** argv)
-{
-	ConditionVariable::ListAll();
-	return 0;
-}
-
-
-static int
-dump_condition_variable(int argc, char** argv)
-{
-	if (argc != 2) {
-		print_debugger_command_usage(argv[0]);
-		return 0;
-	}
-
-	addr_t address = parse_expression(argv[1]);
-	if (address == 0)
-		return 0;
-
-	ConditionVariable* variable = sConditionVariableHash.Lookup((void*)address);
-
-	if (variable == NULL) {
-		// It must be a direct pointer to a condition variable.
-		variable = (ConditionVariable*)address;
-	}
-
-	if (variable != NULL) {
-		variable->Dump();
-
-		set_debug_variable("_cvar", (addr_t)variable);
-		set_debug_variable("_object", (addr_t)variable->Object());
-
-	} else
-		kprintf("no condition variable at or with key %p\n", (void*)address);
-
-	return 0;
-}
-
-
 // #pragma mark - ConditionVariableEntry
 
 
@@ -463,6 +423,9 @@ ConditionVariable::_NotifyLocked(bool all, status_t result)
 }
 
 
+// #pragma mark -
+
+
 /*static*/ void
 ConditionVariable::ListAll()
 {
@@ -494,6 +457,46 @@ ConditionVariable::Dump() const
 }
 
 
+static int
+list_condition_variables(int argc, char** argv)
+{
+	ConditionVariable::ListAll();
+	return 0;
+}
+
+
+static int
+dump_condition_variable(int argc, char** argv)
+{
+	if (argc != 2) {
+		print_debugger_command_usage(argv[0]);
+		return 0;
+	}
+
+	addr_t address = parse_expression(argv[1]);
+	if (address == 0)
+		return 0;
+
+	ConditionVariable* variable = sConditionVariableHash.Lookup((void*)address);
+
+	if (variable == NULL) {
+		// It must be a direct pointer to a condition variable.
+		variable = (ConditionVariable*)address;
+	}
+
+	if (variable != NULL) {
+		variable->Dump();
+
+		set_debug_variable("_cvar", (addr_t)variable);
+		set_debug_variable("_object", (addr_t)variable->Object());
+
+	} else
+		kprintf("no condition variable at or with key %p\n", (void*)address);
+
+	return 0;
+}
+
+
 // #pragma mark -
 
 
@@ -517,5 +520,20 @@ condition_variable_init()
 	add_debugger_command_etc("cvars", &list_condition_variables,
 		"List condition variables",
 		"\n"
-		"Lists all existing condition variables\n", 0);
+		"Lists all published condition variables\n", 0);
+}
+
+
+ssize_t
+debug_condition_variable_type_strlcpy(ConditionVariable* cvar, char* name, size_t size)
+{
+	const int32 typePointerOffset = offsetof(ConditionVariable, fObjectType);
+
+	const char* pointer;
+	status_t status = debug_memcpy(B_CURRENT_TEAM, &pointer,
+		(int8*)cvar + typePointerOffset, sizeof(const char*));
+	if (status != B_OK)
+		return status;
+
+	return debug_strlcpy(B_CURRENT_TEAM, name, pointer, size);
 }

src/system/kernel/thread.cpp

@@ -1701,8 +1701,16 @@ _dump_thread_info(Thread *thread, bool shortInfo)
 				}
 
 				case THREAD_BLOCK_TYPE_CONDITION_VARIABLE:
-					kprintf("cvar      %p   ", thread->wait.object);
+				{
+					char name[5];
+					ssize_t length = debug_condition_variable_type_strlcpy(
+						(ConditionVariable*)thread->wait.object, name, sizeof(name));
+					if (length > 0)
+						kprintf("cvar:%*s %p   ", 4, name, thread->wait.object);
+					else
+						kprintf("cvar      %p   ", thread->wait.object);
 					break;
+				}
 
 				case THREAD_BLOCK_TYPE_SNOOZE:
 					kprintf("%*s", B_PRINTF_POINTER_WIDTH + 15, "");