[PATCH, v3] Expect SI_KERNEL or TRAP_BRKPT si_code values for MIPS breakpoint traps

[Luis Machado <lgustavo at codesourcery dot com>]

This is v3 of the patch, which adds a comment to how MIPS' kernel handles/will
handle si_code values.

Pedro, Maciej, does this look reasonable? Should i expand the comment with
any other bits of information?

--

While doing some MIPS/Linux tests, i've found a number of threading tests
failing due to spurious SIGTRAP's. Turns out those spurious SIGTRAP's were
actually software breakpoint hits that were supposed to be handled silently by
GDB/GDBserver, returning a swbreak event.

gdb.threads/continue-pending-status.exp is one of the testcases that show this
behavior.

--

Breakpoint 1, main () at gdb.threads/continue-pending-status.c:44^M
44        pthread_barrier_init (&barrier, NULL, NUM_THREADS);^M
(gdb) b continue-pending-status.c:36^M
Breakpoint 2 at 0x400a04: file gdb.threads/continue-pending-status.c, line 36.^M
(gdb) PASS: gdb.threads/continue-pending-status.exp: attempt 0: set break in tight loop
continue^M
Continuing.^M
[New Thread 5850]^M
[New Thread 5851]^M
[Switching to Thread 5850]^M
^M
Breakpoint 2, thread_function (arg=0x0) at gdb.threads/continue-pending-status.c:36^M
36        while (1); /* break here */^M
(gdb) PASS: gdb.threads/continue-pending-status.exp: attempt 0: continue to tight loop
print /x $_thread^M
$1 = 0x2^M
(gdb) PASS: gdb.threads/continue-pending-status.exp: attempt 0: get thread number
thread 3^M
[Switching to thread 3 (Thread 5851)]^M
36        while (1); /* break here */^M
(gdb) PASS: gdb.threads/continue-pending-status.exp: attempt 0: switch to non-event thread
delete breakpoints^M
Delete all breakpoints? (y or n) y^M
(gdb) info breakpoints^M
No breakpoints or watchpoints.^M
(gdb) continue^M
Continuing.^M
^M
Program received signal SIGTRAP, Trace/breakpoint trap.^M

<<<< This SIGTRAP was a pending breakpoint event that wasn't supposed to cause
<<<< a stop, but gdbserver did not figure out this was a breakpoint hit.

PASS: gdb.threads/continue-pending-status.exp: attempt 0: continue for ctrl-c
thread_function (arg=0x0) at gdb.threads/continue-pending-status.c:36^M
36        while (1); /* break here */^M
(gdb) FAIL: gdb.threads/continue-pending-status.exp: attempt 0: caught interrupt

--

I tracked this down to the lack of a proper definition of what MIPS' kernel
returns in the si_code for a software breakpoint trap.

Further discussion with MIPS maintainers showed that, historically, MIPS
kernels have never set a proper si_code and thus they use the default value of
SI_KERNEL.

There are plans to update the MIPS kernel to provide more meaningful si_code
values though, so we should expect both SI_KERNEL and TRAP_BRKPT from now
on, as GDB will handle both correctly, like powerpc.

With the following patch i have cleaner results for thread tests on
MIPS/Linux.

Regression-tested on a few MIPS boards.

OK?

gdb/ChangeLog:

2016-02-20  Luis Machado  <lgustavo@codesourcery.com>

  * nat/linux-ptrace.h: Check for both SI_KERNEL and TRAP_BRKPT si_code for
    MIPS.
---
 gdb/nat/linux-ptrace.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
index ba58717..72b32b1 100644
--- a/gdb/nat/linux-ptrace.h
+++ b/gdb/nat/linux-ptrace.h
@@ -140,11 +140,16 @@ struct buffer;
    in SPU code on a Cell/B.E.  However, SI_KERNEL is never seen
    on a SIGTRAP for any other reason.
 
+   The MIPS kernel uses the default si_code of SI_KERNEL for software
+   breakpoints, hardware watchpoints and SIGTRAP's in general.  There are
+   plans to start using a si_code value of TRAP_BRKPT in the kernel, so
+   we check for both values.
+
    The generic Linux target code should use GDB_ARCH_IS_TRAP_BRKPT
    instead of TRAP_BRKPT to abstract out these peculiarities.  */
 #if defined __i386__ || defined __x86_64__
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == SI_KERNEL)
-#elif defined __powerpc__
+#elif defined __powerpc__ || defined __mips__
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == SI_KERNEL || (X) == TRAP_BRKPT)
 #else
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
-- 
1.9.1