scripts/gdb: fix kgdb probing on single-core systems

Patch series "scripts/gdb: Fixes related to lx_per_cpu()".

These patches (1) fix kgdb detection on systems featuring a single CPU and
(2) update the documentation to reflect the current usage of lx_per_cpu()
and update an outdated example of its usage.


This patch (of 2):

When requested the list of threads via qfThreadInfo, gdb_cmd_query in
kernel/debug/gdbstub.c first returns "shadow" threads for CPUs followed by
the actual tasks in the system.  Extended qThreadExtraInfo queries yield
"shadowCPU%d" as the name for the CPU core threads.

This behavior is used by get_gdbserver_type() to probe for KGDB by
matching the name for the thread 2 against "shadowCPU".  This breaks down
on single-core systems, where thread 2 is the first nonshadow thread. 
Request the name for thread 1 instead.

As GDB assigns thread IDs in the order of their appearance, it is safe to
assume shadowCPU0 at ID 1 as long as CPU0 is not hotplugged.

Before:

(gdb) info threads
  Id   Target Id                      Frame
  1    Thread 4294967294 (shadowCPU0) kgdb_breakpoint ()
* 2    Thread 1 (swapper/0)           kgdb_breakpoint ()
  3    Thread 2 (kthreadd)            0x0000000000000000 in ?? ()
  ...
(gdb) p $lx_current().comm
Sorry, obtaining the current CPU is not yet supported with this gdb server.

After:

(gdb) info threads
  Id   Target Id                      Frame
  1    Thread 4294967294 (shadowCPU0) kgdb_breakpoint ()
* 2    Thread 1 (swapper/0)           kgdb_breakpoint ()
  3    Thread 2 (kthreadd)            0x0000000000000000 in ?? ()
  ...
(gdb) p $lx_current().comm
$1 = "swapper/0\000\000\000\000\000\000"

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Illia Ostapyshyn <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Brendan Jackman <[email protected]>
Cc: Dongliang Mu <[email protected]>
Cc: Florian Rommel <[email protected]>
Cc: Hu Haowen <[email protected]>
Cc: Jan Kiszka <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kieran Bingham <[email protected]>
Cc: Yanteng Si <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: iostapyshyn

scripts/gdb/linux/utils.py

@@ -200,7 +200,7 @@ def probe_qemu():
 
     def probe_kgdb():
         try:
-            thread_info = gdb.execute("info thread 2", to_string=True)
+            thread_info = gdb.execute("info thread 1", to_string=True)
             return "shadowCPU" in thread_info
         except gdb.error:
             return False