scripts/gdb: add lx_thread_info_by_pid helper

The tasks module already provides helpers to find the task struct by
pid, and the thread_info by task struct; however this is cumbersome to
utilise on the gdb commandline.

Wrap these two functionalities together in an extra single helper to
allow exploring the thread info, from a PID value

Link: http://lkml.kernel.org/r/dadc5667f053ec811eb3e3033d99d937fedbc93b.1462865983.git.jan.kiszka@siemens.com
Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/gdb/linux/tasks.py

@@ -114,3 +114,22 @@ variable."""
 
 
 LxThreadInfoFunc()
+
+
+class LxThreadInfoByPidFunc (gdb.Function):
+    """Calculate Linux thread_info from task variable found by pid
+
+$lx_thread_info_by_pid(PID): Given PID, return the corresponding thread_info
+variable."""
+
+    def __init__(self):
+        super(LxThreadInfoByPidFunc, self).__init__("lx_thread_info_by_pid")
+
+    def invoke(self, pid):
+        task = get_task_by_pid(pid)
+        if task:
+            return get_thread_info(task.dereference())
+        else:
+            raise gdb.GdbError("No task of PID " + str(pid))
+
+LxThreadInfoByPidFunc()