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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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Barry Song 2021-05-06 18:05:06 -07:00 committed by Linus Torvalds
parent 23921540d2
commit dc9586823f
2 changed files with 9 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
Documentation/dev-tools/gdb-kernel-debugging.rst
View File

@ -114,7 +114,7 @@ Examples of using the Linux-provided gdb helpers
[ 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

10
scripts/gdb/linux/cpus.py
View File

@ -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 the current context is used."""
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()