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/[email protected] Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Barry Song <[email protected]> Cc: Jan Kiszka <[email protected]> Cc: Kieran Bingham <[email protected]> Cc: Jonathan Corbet <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
josf · May 7, 2021 · dc95868 · dc95868
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commit dc95868
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diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst b/Documentation/dev-tools/gdb-kernel-debugging.rst
@@ -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

diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
@@ -156,6 +156,13 @@ def invoke(self, var_name, cpu=-1):
 
 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 @@ def __init__(self):
         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()