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Re: [PATCH] Fix problems with finishing a dummy function call on simulators.


On 06/09/2015 02:51 PM, Pedro Alves wrote:
On 06/09/2015 04:00 PM, Luis Machado wrote:
This is in line with what was done by Joel's patch here:

https://sourceware.org/ml/gdb-patches/2014-11/msg00478.html

And it also answers Pedro's question about whether this is specific to SPARC
QEMU or not.  This indeed seems to affect multiple QEMU targets and also other
simulators (proprietary).

Sounds like a different issue, although related.


I ran into this weird issue of not being able to "finish" an inferior function
call. It looks as if the program is running away, but it really is stuck
somewhere.  "finish" still works fine for regular functions not called manually
by GDB.

Sounds like that would fail on SPARC qemu as well.


I tracked this failure down to GDB having both a bp_call_dummy and bp_finish in
its breakpoint list. As a result of one not being considered permanent and the
other considered permanent, GDB will not issue a Z packet to force the insertion
of that location's breakpoint, confusing the simulator that does not know how
to deal properly with these permanent breakpoints that GDB inserted beforehand.

The attached patch fixes this, though i'm inclined to say we could probably
check if both bp_call_dummy and bp_finish are present and force the
insertion of that location's breakpoint.  It isn't clear to me where exactly that
check would go or if it would be cleaner than checking that information in
the same function Joel used.

I see no regressions on x86-64 and it fixes a bunch of failures for simulator
targets we use (MIPS and PowerPC to name two).

If it happens that you "finish" from a normal function, and the finish
breakpoint ends up on top of a real permanent breakpoint, then this patch
will make us end up inserting a breakpoint on top of that permanent
breakpoint.  I don't see what's special about finish breakpoints;
it's the address (dummy breakpoint location) that is special.  It very much
sounds like that any kind of breakpoint that is placed on top of the dummy
breakpoint ends up with the same issue.  E.g., if you stepi out of
the called function, with a software single-step breakpoint, sounds like
GDB will miss inserting the software step breakpoint because that's
at the same address as the dummy breakpoint.

As a data point, I assume that GDB is considering the non-permanent
dummy breakpoint a duplicate of the permanent finish breakpoint and
then none ends up inserted.  Is that right?

Not exactly sure what to do here.  Maybe we should stop considering
permanent and non-permanent breakpoints at the same address as
duplicates.  That should result in GDB inserting the non-permanent
one, I think.  Or we could get stop marking permanent breakpoints
as always inserted, and let normal breakpoints insert on top of
permanent breakpoints normally.  See also:
  https://sourceware.org/ml/gdb-patches/2015-03/msg00174.html

I gave the strategy of not marking permanent breakpoints/locations as inserted a try, and it fixes the simulator problems i've been seeing with the permanent breakpoint locations.

One strange side effect of this change on my local machine (x86-64) is that gdb.threads/attach-many-short-lived-threads.exp gives me PASS instead of FAIL when always-inserted mode is ON. I didn't investigate this further though. Is it known that this testcase is affected by permanent breakpoint locations?

For example:

XFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 5: attach
(EPERM)
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: no new
threads
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: set
breakpoint always-inserted on
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: break
break_fn
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: break at
break_fn: 1
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: break at
break_fn: 2
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: break at
break_fn: 3
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: reset
timer in the inferior
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: print
seconds_left
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: detach
PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: set
breakpoint always-inserted off

Is this patch what you had in mind?

Luis
2015-06-16  Luis Machado  <lgustavo@codesourcery.com>

	* breakpoint.c (make_breakpoint_permanent): Expand comment.
	Don't mark permanent locations as inserted.
	(add_location_to_breakpoint): Likewise
	(update_global_location_list): Don't error out if a permanent
	breakpoint is not marked inserted.
	Don't error out if a non-permanent breakpoint location is inserted on
	top of a permanent breakpoint.

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index eb3df02..768ce59 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -7440,15 +7440,16 @@ make_breakpoint_permanent (struct breakpoint *b)
   struct bp_location *bl;
 
   /* By definition, permanent breakpoints are already present in the
-     code.  Mark all locations as inserted.  For now,
-     make_breakpoint_permanent is called in just one place, so it's
-     hard to say if it's reasonable to have permanent breakpoint with
-     multiple locations or not, but it's easy to implement.  */
+     code.  For now, make_breakpoint_permanent is called in just one place, so
+     it's hard to say if it's reasonable to have permanent breakpoint with
+     multiple locations or not, but it's easy to implement.
+
+     Permanent breakpoints are not marked as inserted so we allow other
+     non-permanent locations at the same address to be inserted on top
+     of it.  This is required due to some targets, simulators mostly, not
+     dealing properly with hardwired breakpoints in the code.  */
   for (bl = b->loc; bl; bl = bl->next)
-    {
-      bl->permanent = 1;
-      bl->inserted = 1;
-    }
+    bl->permanent = 1;
 }
 
 /* Call this routine when stepping and nexting to enable a breakpoint
@@ -8918,11 +8919,10 @@ add_location_to_breakpoint (struct breakpoint *b,
   set_breakpoint_location_function (loc,
 				    sal->explicit_pc || sal->explicit_line);
 
+  /* See comment in make_breakpoint_permanent for the reason why we don't mark
+     permanent breakpoints as always inserted.  */
   if (bp_loc_is_permanent (loc))
-    {
-      loc->inserted = 1;
-      loc->permanent = 1;
-    }
+    loc->permanent = 1;
 
   return loc;
 }
@@ -12438,12 +12438,6 @@ update_global_location_list (enum ugll_insert_mode insert_mode)
 	  continue;
 	}
 
-      /* Permanent breakpoint should always be inserted.  */
-      if (loc->permanent && ! loc->inserted)
-	internal_error (__FILE__, __LINE__,
-			_("allegedly permanent breakpoint is not "
-			"actually inserted"));
-
       if (b->type == bp_hardware_watchpoint)
 	loc_first_p = &wp_loc_first;
       else if (b->type == bp_read_watchpoint)
@@ -12479,12 +12473,6 @@ update_global_location_list (enum ugll_insert_mode insert_mode)
 
       /* Clear the condition modification flag.  */
       loc->condition_changed = condition_unchanged;
-
-      if (loc->inserted && !loc->permanent
-	  && (*loc_first_p)->permanent)
-	internal_error (__FILE__, __LINE__,
-			_("another breakpoint was inserted on top of "
-			"a permanent breakpoint"));
     }
 
   if (insert_mode == UGLL_INSERT || breakpoints_should_be_inserted_now ())

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