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Re: [PATCH 2/2] Make breakpoint handling in record-full idempotent

On 04/04/2016 03:38 AM, Yao Qi wrote:
Some test fails in gdb.reverse/break-reverse.exp on arm-linux lead me
seeing the following error message,

Cannot remove breakpoints because program is no longer writable.^M
Further execution is probably impossible.^M
Breakpoint 3, bar () at /home/yao/SourceCode/gnu/gdb/git/gdb/testsuite/gdb.reverse/break-reverse.c:22^M
22        xyz = 2; /* break in bar */^M
(gdb) PASS: gdb.reverse/break-reverse.exp: continue to breakpoint: bar backward

this is caused by two entries in record_full_breakpoints, and their addr
is the same, but in_target_beneath is different.

during the record, we do continue,

infrun: clear_proceed_status_thread (Thread 13772.13772)
infrun: proceed (addr=0xffffffff, signal=GDB_SIGNAL_DEFAULT)
infrun: step-over queue now empty
infrun: resuming [Thread 13772.13772] for step-over
infrun: skipping breakpoint: stepping past insn at: 0x8620
Sending packet: $Z0,85f4,4#1d...Packet received: OK  <----
Sending packet: $vCont;c#a8...infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [process -1],
infrun:   status->kind = ignore
infrun: prepare_to_wait
infrun: target_wait (-1.0.0, status) =
infrun:   -1.0.0 [process -1],
infrun:   status->kind = ignore
infrun: prepare_to_wait
Packet received: T05swbreak:;0b:9cf5ffbe;0d:9cf5ffbe;0f:f4850000;thread:p35cc.35cc;core:1;
Sending packet: $Z0,85f4,4#1d...Packet received: OK <-----
Sending packet: $z0,85f4,4#3d...Packet received: OK <-----

we can see breakpoint on 0x85f4 are inserted *twice*, but only removed
once.  That is fine to remote target, because Z/z packets are
idempotent, but there is a leftover in record_full_breakpoints
in record-full target.  The flow can be described as below,

                                 record_full_breakpoints   remote target
   forward execution, continue,    in_target_beneath 1     breakpoint inserted
   insert breakpoints on 0x85f4    in_target_beneath 1

   program stops,
   remove breakpoint on 0x85f4     in_target_beneath 1     breakpoint removed

   reverse execution, continue,    in_target_beneath 1     none is requested
   insert breakpoints on 0x85f4,   in_target_beneath 0

   program stops,
   remote breakpoint on 0x85f4,    in_target_beneath 0     request to remove,
                                                           but GDBserver
							  doesn't know

now, the question is why breakoint on 0x85f4 is inserted twice?  One
is the normal breakpoint, and the other is the single step breakpoint.
GDB inserts single step breakpoint to do single step.  When program
stops at 0x85f4, both of them are set on 0x85f4, and GDB deletes
single step breakpoint, so in update_global_location_list, this
breakpoint location is no longer found, GDB call
force_breakpoint_reinsertion to mark it condition_updated, and insert
it again.

The reason force_breakpoint_reinsertion is called to update the
conditions in the target side, because the conditions may be
changed.  My original fix is to not call force_breakpoint_reinsertion
if OLD_LOC->cond is NULL, but it is not correct if another location
on the same address has condition, GDB doesn't produce condition for
target side, but GDB should do.

Then, I change my mind back to make record-full handling breakpoint
idempotent, to align with remote target.  Before insert a new entry
into record_full_breakpoints, look for existing one on the same
address first.  I also add an assert on
"bp->in_target_beneath == in_target_beneath", to be safer.


2016-04-04  Yao Qi  <>

	* record-full.c (record_full_insert_breakpoint): Return
	early if entry on the address is found in
  gdb/record-full.c | 15 +++++++++++++++
  1 file changed, 15 insertions(+)

diff --git a/gdb/record-full.c b/gdb/record-full.c
index 066a8e7..9e7694e 100644
--- a/gdb/record-full.c
+++ b/gdb/record-full.c
@@ -1650,6 +1650,7 @@ record_full_insert_breakpoint (struct target_ops *ops,
    struct record_full_breakpoint *bp;
    int in_target_beneath = 0;
+  int ix;

@@ -1681,6 +1682,20 @@ record_full_insert_breakpoint (struct target_ops *ops,
        bp_tgt->placed_size = bplen;

+  /* Find any existing entries.  */

Should this say...

/* Make sure there are no duplicate breakpoint entries.  */

... instead?

Otherwise it looks good to me.

+  for (ix = 0;
+       VEC_iterate (record_full_breakpoint_p,
+		    record_full_breakpoints, ix, bp);
+       ++ix)
+    {
+      if (bp->addr == bp_tgt->placed_address
+	  && bp->address_space == bp_tgt->placed_address_space)
+	{
+	  gdb_assert (bp->in_target_beneath == in_target_beneath);
+	  return 0;
+	}
+    }
    bp = XNEW (struct record_full_breakpoint);
    bp->addr = bp_tgt->placed_address;
    bp->address_space = bp_tgt->placed_address_space;

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