Date: Wed, 6 Apr 2011 08:45:11 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Justin Hibbits <chmeeedalf@gmail.com>, Navdeep Parhar <nparhar@gmail.com>, FreeBSD Current <current@freebsd.org> Subject: Re: KGDB stack traces in the kernel. Message-ID: <201104060845.11771.jhb@freebsd.org> In-Reply-To: <BANLkTinPdVUac6YGwf4GvOE7gtndbQERaQ@mail.gmail.com> References: <4D9A4CE5.5090900@freebsd.org> <4D9B7C92.6030901@freebsd.org> <BANLkTinPdVUac6YGwf4GvOE7gtndbQERaQ@mail.gmail.com>
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On Tuesday, April 05, 2011 4:35:44 pm Navdeep Parhar wrote:
> On Tue, Apr 5, 2011 at 1:33 PM, Julian Elischer <julian@freebsd.org> wrote:
> > On 4/4/11 6:04 PM, Justin Hibbits wrote:
> >>
> >> On Apr 4, 2011, at 6:57 PM, Julian Elischer wrote:
> >>>
> >>> is there anyone here with enough gdb/kgdb source experience to know what
> >>> we would need to put on the stack at fork_exit() to make it stop when it
> >>> gets there?
> >>>
> >>> not only is it annoying but it slows down debugging because kgdb and the
> >>> ddd
> >>> front end ask for stacks a LOT. sometimes it actually just hangs as the
> >>> stack
> >>> goes into a loop and never ends.
> >>>
> >>> I had a quick look but didn't spot how gdb decides it has reached the
end
> >>> of a stack.
> >>>
> >>> Julian
> >>
> >> From my experience, it checks for a NULL stack chain pointer. Once that
> >> reaches NULL, it's the end of the stack.
> >>
> >> - Justin
> >>
> > I'll try adding NULL when we build the intial stack up.
> > :-)
>
> What does ddb do? It always seems to get this stuff correct.
ddb knows to stop when it gets to a non-kernel address, and it uses string
compares on function names to identify trap frames. For example in
sys/amd64/amd64/db_trace.c:
if (strcmp(name, "calltrap") == 0 ||
strcmp(name, "fork_trampoline") == 0 ||
strcmp(name, "nmi_calltrap") == 0 ||
strcmp(name, "Xdblfault") == 0)
frame_type = TRAP;
Hah, kgdb just needs to be updated (this is from trgt_amd64.c):
const struct frame_unwind *
kgdb_trgt_trapframe_sniffer(struct frame_info *next_frame)
{
char *pname;
CORE_ADDR pc;
pc = frame_pc_unwind(next_frame);
pname = NULL;
find_pc_partial_function(pc, &pname, NULL, NULL);
if (pname == NULL)
return (NULL);
if (strcmp(pname, "calltrap") == 0 ||
strcmp(pname, "nmi_calltrap") == 0 ||
(pname[0] == 'X' && pname[1] != '_'))
return (&kgdb_trgt_trapframe_unwind);
/* printf("%s: %lx =%s\n", __func__, pc, pname); */
return (NULL);
}
Can probably just add 'fork_trampoline' to that conditional. I think i386
needs a similar fix in kgdb. Not sure about other architectures:
Index: trgt_amd64.c
===================================================================
--- trgt_amd64.c (revision 220190)
+++ trgt_amd64.c (working copy)
@@ -184,6 +184,7 @@
if (pname == NULL)
return (NULL);
if (strcmp(pname, "calltrap") == 0 ||
+ strcmp(pname, "fork_trampoline") == 0 ||
strcmp(pname, "nmi_calltrap") == 0 ||
(pname[0] == 'X' && pname[1] != '_'))
return (&kgdb_trgt_trapframe_unwind);
Index: trgt_i386.c
===================================================================
--- trgt_i386.c (revision 220190)
+++ trgt_i386.c (working copy)
@@ -374,6 +374,7 @@
if (strcmp(pname, "dblfault_handler") == 0)
return (&kgdb_trgt_dblfault_unwind);
if (strcmp(pname, "calltrap") == 0 ||
+ strcmp(pname, "fork_trampoline") == 0 ||
(pname[0] == 'X' && pname[1] != '_'))
return (&kgdb_trgt_trapframe_unwind);
/* printf("%s: %llx =%s\n", __func__, pc, pname); */
--
John Baldwin
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