Re: [PATCH]: Fix PR19061, gdb hangs/spins-on-cpu when debugging any program on Alpha

[Simon Marchi <simon.marchi@ericsson.com>]

On 2018-05-31 10:36 AM, Simon Marchi wrote:
> On 2018-05-31 05:23, Uros Bizjak wrote:
>> On Wed, May 30, 2018 at 4:53 PM, Simon Marchi <simon.marchi@ericsson.com> wrote:
>>> Does Alpha even have hardware breakpoints?  If not, I would suggest
>>> defining GDB_ARCH_IS_TRAP_HWBKPT to 0 for __alpha__.  It would get
>>> rid of the error, and be more exact (no si_code can mean "hardware
>>> breakpoint" on alpha).
>>
>> I didn't find any mentions of hardware breakpoint support in Alpha
>> Architecture Handbook v.4.
>>
>> Please note that save_stop_reason from linux-nat.c has some code to
>> detect ambigous si_code, where si_code is both TRAP_BRKPT and
>> TRAP_HWBKPT, and returns TARGET_STOPPED_BY_SW_BREAKPOINT in this case.
>> So, if we don't define anything for __alpha__ in nat/linux-ptrace.h,
>> we get the same result.
> 
> TRAP_BKPT and TRAP_HWBKPT are different, so this will never happen with the default definitions of GDB_ARCH_IS_TRAP_BRKPT/GDB_ARCH_IS_TRAP_HWBKPT.  Since the kernel for alpha never sets si_code to TRAP_HWBKPT, it means we'll never get in to the HW breakpoint branch, which is fine.
> 
> In short, the patch LGTM.
> 
>> I have tested the attached patch, and resulting gdb works OK for me.
> 
> Thanks!
> 
> Simon

Since Uros doesn't have write access, I wrote a commit log and pushed
the patch.

Simon


From 8a60efe714e636c9f958058a8dfb12de81bdcbfa Mon Sep 17 00:00:00 2001
From: Uros Bizjak <ubizjak@gmail.com>
Date: Thu, 31 May 2018 11:18:02 -0400
Subject: [PATCH] Fix Alpha native GDB build

[Commit log by Simon Marchi]

I get this error:

  CXX    linux-nat.o
/home/simark/src/binutils-gdb/gdb/linux-nat.c: In function 'void save_stop_reason(lwp_info*)':
/home/simark/src/binutils-gdb/gdb/linux-nat.c:2718:9: error: duplicated 'if' condition [-Werror=duplicated-cond]
    else if (GDB_ARCH_IS_TRAP_HWBKPT (siginfo.si_code))
         ^~
In file included from /home/simark/src/binutils-gdb/gdb/linux-nat.c:31:0:
/home/simark/src/binutils-gdb/gdb/nat/linux-ptrace.h:173:41: note: previously used here
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
                                    ~~~~~^~~~~~~~~~~~~~
/home/simark/src/binutils-gdb/gdb/linux-nat.c:2709:13: note: in expansion of macro 'GDB_ARCH_IS_TRAP_BRKPT'
    else if (GDB_ARCH_IS_TRAP_BRKPT (siginfo.si_code))
             ^~~~~~~~~~~~~~~~~~~~~~

For Alpha, we currently define GDB_ARCH_IS_TRAP_BRKPT and
GDB_ARCH_IS_TRAP_HWBKPT both to ((X) == TRAP_BRKPT), which causes the
two if branches to be duplicated.

Alpha doesn't have hardware breakpoints, so the Linux kernel for Alpha
never sets si_code to TRAP_HWBKPT.  We can just remove the special
definitions of these macros for __alpha__ and rely on the default ones.
Since the kernel will never report TRAP_HWBKPT, we will just never enter
the "hardware breakpoint" branch on Alpha (which is fine since it
doesn't have them).

gdb/ChangeLog:

	* nat/linux-ptrace.h [__alpha__]
	(GDB_ARCH_IS_TRAP_BRKPT, GDB_ARCH_IS_TRAP_HWBKPT): Remove
	definitions.
---
 gdb/ChangeLog          | 6 ++++++
 gdb/nat/linux-ptrace.h | 5 -----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c6c862f..8af7e67 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2018-05-31  Uros Bizjak  <ubizjak@gmail.com>
+
+	* nat/linux-ptrace.h [__alpha__]
+	(GDB_ARCH_IS_TRAP_BRKPT, GDB_ARCH_IS_TRAP_HWBKPT): Remove
+	definitions.
+
 2018-05-31  Maciej W. Rozycki  <macro@mips.com>

 	* arch-utils.c (gdbarch_info_fill): Set `default_byte_order' to
diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
index dc180fb..98b44a8 100644
--- a/gdb/nat/linux-ptrace.h
+++ b/gdb/nat/linux-ptrace.h
@@ -156,8 +156,6 @@ struct buffer;
    Beginning with Linux 4.6, the MIPS port reports proper TRAP_BRKPT and
    TRAP_HWBKPT codes, so we also match them.

-   The Alpha kernel uses TRAP_BRKPT for all traps.
-
    The generic Linux target code should use GDB_ARCH_IS_TRAP_* instead
    of TRAP_* to abstract out these peculiarities.  */
 #if defined __i386__ || defined __x86_64__
@@ -169,9 +167,6 @@ struct buffer;
 #elif defined __mips__
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == SI_KERNEL || (X) == TRAP_BRKPT)
 # define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == SI_KERNEL || (X) == TRAP_HWBKPT)
-#elif defined __alpha__
-# define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
-# define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == TRAP_BRKPT)
 #else
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
 # define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == TRAP_HWBKPT)
-- 
2.7.4