Re: [RFC] "single step" atomic instruction sequences as a whole on PPC

Ulrich Weigand
2007-05-10T21:31:06Z

Hello Emi-san,

> When that multiple SIGTRAP events occured, GDB selects one event and
> cancels the other if the cause of SIGTRAP is a breakpoint hit, or just
> leave it pended. The procedure will be done by `cancel_breakpoints_callback'
> in linux-nat.c. And the pended events will be deteced and noticed
> when the next time the target resumes.
>
> The probrem is that GDB doesn't check if the breakpoint is inserted
> for software single stepping when cancelling the trap event: when the
> event occured by a software single step breakpoint is not selected,
> GDB would not cancel it but leave it pended.

This is cancel_breakpoints_callback in linux-nat.c, right?

> When the next time the target resumes, GDB restores the pended event.
> But if you have removed the watchpoint that the target get stopped by
> before resuming, GDB can never decide the cause of SIGTRAP anymore.
> The session log above shows the phenomenon.

This looks just like a problem we fixed for the combined debugger;
cancel_breakpoints_callback should cancel SIGTRAP events caused by
software single-step breakpoints just the same as those caused by
other breakpoints.

Can you try whether the following patch fixes the problem for you?

Bye,
Ulrich

ChangeLog:

* breakpoint.c (single_step_breakpoint_inserted_here_p): New function.
* breakpoint.h (single_step_breakpoint_inserted_here_p): New prototype.
* linux-nat.c (cancel_breakpoints_callback): Treat software single-
step breakpoints the same way as regular software breakpoints.

diff -urNp gdb-orig/gdb/breakpoint.c gdb-head/gdb/breakpoint.c
--- gdb-orig/gdb/breakpoint.c 2007-05-08 14:37:32.000000000 +0200
+++ gdb-head/gdb/breakpoint.c 2007-05-10 22:34:36.776868666 +0200
@@ -7951,6 +7951,21 @@ remove_single_step_breakpoints (void)
}
}

+int
+single_step_breakpoint_inserted_here_p (CORE_ADDR pc)
+{
+ int i;
+
+ for (i = 0; i < 2; i++)
+ {
+ struct bp_target_info *bp_tgt = single_step_breakpoints[i];
+ if (bp_tgt && bp_tgt->placed_address == pc)
+ return 1;
+ }
+
+ return 0;
+}
+
\f
/* This help string is used for the break, hbreak, tbreak and thbreak commands.
It is defined as a macro to prevent duplication.
diff -urNp gdb-orig/gdb/breakpoint.h gdb-head/gdb/breakpoint.h
--- gdb-orig/gdb/breakpoint.h 2007-04-13 19:13:45.000000000 +0200
+++ gdb-head/gdb/breakpoint.h 2007-05-10 22:34:36.829861036 +0200
@@ -832,6 +832,8 @@ extern int remove_hw_watchpoints (void);
twice before remove is called. */
extern void insert_single_step_breakpoint (CORE_ADDR);
extern void remove_single_step_breakpoints (void);
+extern int single_step_breakpoint_inserted_here_p (CORE_ADDR pc);
+

/* Manage manual breakpoints, separate from the normal chain of
breakpoints. These functions are used in murky target-specific
diff -urNp gdb-orig/gdb/linux-nat.c gdb-head/gdb/linux-nat.c
--- gdb-orig/gdb/linux-nat.c 2007-05-07 02:18:12.000000000 +0200
+++ gdb-head/gdb/linux-nat.c 2007-05-10 22:34:36.860856573 +0200
@@ -1765,21 +1765,26 @@ cancel_breakpoints_callback (struct lwp_
tripped on it. */

if (lp->status != 0
- && WIFSTOPPED (lp->status) && WSTOPSIG (lp->status) == SIGTRAP
- && breakpoint_inserted_here_p (read_pc_pid (lp->ptid) -
- DECR_PC_AFTER_BREAK))
+ && WIFSTOPPED (lp->status) && WSTOPSIG (lp->status) == SIGTRAP)
{
- if (debug_linux_nat)
- fprintf_unfiltered (gdb_stdlog,
- "CBC: Push back breakpoint for %s\n",
- target_pid_to_str (lp->ptid));
+ CORE_ADDR break_pc;
+ break_pc = read_pc_pid (lp->ptid) - DECR_PC_AFTER_BREAK;

- /* Back up the PC if necessary. */
- if (DECR_PC_AFTER_BREAK)
- write_pc_pid (read_pc_pid (lp->ptid) - DECR_PC_AFTER_BREAK, lp->ptid);
+ if (breakpoint_inserted_here_p (break_pc)
+ || single_step_breakpoint_inserted_here_p (break_pc))
+ {
+ if (debug_linux_nat)
+ fprintf_unfiltered (gdb_stdlog,
+ "CBC: Push back breakpoint for %s\n",
+ target_pid_to_str (lp->ptid));

- /* Throw away the SIGTRAP. */
- lp->status = 0;
+ /* Back up the PC if necessary. */
+ if (DECR_PC_AFTER_BREAK)
+ write_pc_pid (break_pc, lp->ptid);
+
+ /* Throw away the SIGTRAP. */
+ lp->status = 0;
+ }
}

return 0;

--
Dr. Ulrich Weigand
GNU Toolchain for Linux on System z and Cell BE
Ulrich.Weigand@de.ibm.com