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Date:   Thu, 06 May 2021 18:05:06 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     akpm@linux-foundation.org, corbet@lwn.net, jan.kiszka@siemens.com,
        kbingham@kernel.org, linux-mm@kvack.org,
        mm-commits@vger.kernel.org, song.bao.hua@hisilicon.com,
        torvalds@linux-foundation.org
Subject:  [patch 59/91] scripts/gdb: document lx_current is only
 supported by x86
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From: Barry Song <song.bao.hua@hisilicon.com>
Subject: scripts/gdb: document lx_current is only supported by x86

Patch series "scripts/gdb: clarify the platforms supporting lx_current and add arm64 support", v2.

lx_current depends on per_cpu current_task variable which exists on x86
only.  so it actually works on x86 only.  the 1st patch documents this
clearly; the 2nd patch adds support for arm64.


This patch (of 2):

x86 is the only architecture which has per_cpu current_task:
arch$ git grep current_task | grep -i per_cpu
x86/include/asm/current.h:DECLARE_PER_CPU(struct task_struct *, current_task);
x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *, current_task) ____cacheline_aligned =
x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task);
x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *, current_task) = &init_task;
x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task);
x86/kernel/smpboot.c:	per_cpu(current_task, cpu) = idle;

On other architectures, lx_current() will lead to a python exception:
(gdb) p $lx_current().pid
Python Exception <class 'gdb.error'> No symbol "current_task" in current context.:
Error occurred in Python: No symbol "current_task" in current context.

To avoid more people struggling and wasting time in other architectures,
document it.

Link: https://lkml.kernel.org/r/20210314203444.15188-1-song.bao.hua@hisilicon.com
Link: https://lkml.kernel.org/r/20210314203444.15188-2-song.bao.hua@hisilicon.com
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/dev-tools/gdb-kernel-debugging.rst |    2 +-
 scripts/gdb/linux/cpus.py                        |   10 ++++++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

--- a/Documentation/dev-tools/gdb-kernel-debugging.rst~scripts-gdb-document-lx_current-is-only-supported-by-x86
+++ a/Documentation/dev-tools/gdb-kernel-debugging.rst
@@ -114,7 +114,7 @@ Examples of using the Linux-provided gdb
     [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
     ....
 
-- Examine fields of the current task struct::
+- Examine fields of the current task struct(supported by x86 only)::
 
     (gdb) p $lx_current().pid
     $1 = 4998
--- a/scripts/gdb/linux/cpus.py~scripts-gdb-document-lx_current-is-only-supported-by-x86
+++ a/scripts/gdb/linux/cpus.py
@@ -156,6 +156,13 @@ Note that VAR has to be quoted as string
 
 PerCpu()
 
+def get_current_task(cpu):
+    if utils.is_target_arch("x86"):
+         var_ptr = gdb.parse_and_eval("&current_task")
+         return per_cpu(var_ptr, cpu).dereference()
+    else:
+        raise gdb.GdbError("Sorry, obtaining the current task is not yet "
+                           "supported with this arch")
 
 class LxCurrentFunc(gdb.Function):
     """Return current task.
@@ -167,8 +174,7 @@ number. If CPU is omitted, the CPU of th
         super(LxCurrentFunc, self).__init__("lx_current")
 
     def invoke(self, cpu=-1):
-        var_ptr = gdb.parse_and_eval("&current_task")
-        return per_cpu(var_ptr, cpu).dereference()
+        return get_current_task(cpu)
 
 
 LxCurrentFunc()
_