4. Debugging with the Memory Pool System¶
Memory management errors are some of the most stubborn and difficult to track down, because the effect so often appears at a distant point in the program that is seemingly unrelated to the cause, and by the time the error is revealed, the information needed to reconstruct the cause has long vanished. Immediately after an overwriting error, the block that overran its bounds is fine, and the block that was overwritten may not be visited for a long time. A failure to fix a reference does not necessarily cause the object pointed to by the missed reference to die immediately: there may be other references to that object, or a garbage collection may be delayed. And even if it does die, the space it occupies may not be re-allocated for some time.
4.1. General debugging advice¶
Compile with debugging information turned on (
-gon the GCC or Clang command line).Build the cool variety of the MPS (by defining the preprocessor constant
CONFIG_VAR_COOL, for example by setting-DCONFIG_VAR_COOLon the GCC or Clang command line). This variety contains many internal consistency checks (including such checks on the critical path, which make it too slow for use in production), and can generate profiling output in the form of the telemetry stream.Prepare a reproducible test case if possible. The MPS may be asynchronous, but it is deterministic, so in single-threaded applications you should be able to get consistent results. (But you need to beware of address space layout randomization: if you perform computation based on the addresses of objects, for example, hashing objects by their address, then ASLR will cause your hash tables to be laid out differently on each run, which may affect the order of memory management operations.)
A fact that assists with reproducibility is that the more frequently the collector runs, the sooner and more reliably errors are discovered. So if you have a bug that’s hard to reproduce, or which manifests itself in different ways on different runs, you may be able to provoke it more reliably, or get a more consistent result, by having a mode for testing in which you run frequent collections (by calling
mps_arena_collect()followed bymps_arena_release()), perhaps as frequently as every allocation.Run your test case inside the debugger. Use
assertandabortin your error handler (rather thanexit) so that you can enter the debugger with the contents of the control stack available for inspection.You may need to make sure that the debugger isn’t entered on barrier (1) hits (because the MPS uses barriers to protect parts of memory, and barrier hits are common and expected).
If you are using GDB on Linux or FreeBSD, run this command:
handle SIGSEGV pass nostop noprint
On OS X barrier hits do not use signals and so do not enter the debugger.
(On these operating systems, you can add these commands to your
.gdbinitif you always want them to be run.)
4.2. Example: underscanning¶
An easy mistake to make is to omit to fix a reference when scanning a formatted object. For example, in the Scheme interpreter’s scan method, I might have forgotten to fix the first element of a pair:
case TYPE_PAIR:
/* oops, forgot: FIX(CAR(obj)); */
FIX(CDR(obj));
base = (char *)base + ALIGN_OBJ(sizeof(pair_s));
break;
This means that as far as the MPS is concerned, the first element of
the pair is unreachable and so dead, so after
collecting the region of memory containing this object, the space will
be reused for other objects. So CAR(obj) might end up pointing to
the start of a valid object (but the wrong one), or to the middle of a
valid object, or to an unused region of memory, or into an MPS
internal control structure.
The reproducible test case is simple. Run a garbage collection by
calling (gc) and then evaluate any expression:
$ gdb ./scheme
GNU gdb 6.3.50-20050815 (Apple version gdb-1820) (Sat Jun 16 02:40:11 UTC 2012)
(gdb) run
Starting program: example/scheme/scheme
Reading symbols for shared libraries +............................. done
MPS Toy Scheme Example
7944, 0> (gc)
Collection started.
Why: Client requests: immediate full collection.
Clock: 11357
Collection finished.
live 1888
condemned 7968
not_condemned 0
clock: 12008
7968, 1> foo
Assertion failed: (TYPE(frame) == TYPE_PAIR), function lookup_in_frame, file scheme.c, line 1065.
Program received signal SIGABRT, Aborted.
0x00007fff91aeed46 in __kill ()
What’s going on?
(gdb) backtrace
#0 0x00007fff91aeed46 in __kill ()
#1 0x00007fff90509df0 in abort ()
#2 0x00007fff9050ae2a in __assert_rtn ()
#3 0x0000000100003f55 in lookup_in_frame (frame=0x1003fa7d0, symbol=0x1003faf20) at scheme.c:1066
#4 0x0000000100003ea6 in lookup (env=0x1003fb130, symbol=0x1003faf20) at scheme.c:1087
#5 0x000000010000341f in eval (env=0x1003fb130, op_env=0x1003fb148, exp=0x1003faf20) at scheme.c:1135
#6 0x000000010000261b in start (p=0x0, s=0) at scheme.c:3204
#7 0x0000000100011ded in ProtTramp (resultReturn=0x7fff5fbff7d0, f=0x100002130 <start>, p=0x0, s=0) at protix.c:132
#8 0x0000000100001ef7 in main (argc=1, argv=0x7fff5fbff830) at scheme.c:3314
(gdb) frame 4
#4 0x0000000100003ea6 in lookup (env=0x1003fb130, symbol=0x1003faf20) at scheme.c:1087
1086 binding = lookup_in_frame(CAR(env), symbol);
(gdb) print (char *)symbol->symbol.string
$1 = 0x1003faf30 "foo"
The backtrace shows that the interpreter is in the middle of looking
up the symbol foo in the environment. The Scheme interpreter
implements the environment as a list of frames, each of which is a
list of bindings, each binding being a pair of a symbol and its
value, as shown here:
The environment data structure in the Scheme interpreter.¶
In this case, because the evaluation is taking place at top level, there is only one frame in the environment (the global frame). And it’s this frame that’s corrupt:
(gdb) frame 3
#3 0x0000000100003f55 in lookup_in_frame (frame=0x1003fa7d0, symbol=0x1003faf20) at scheme.c:1066
1066 assert(TYPE(frame) == TYPE_PAIR);
(gdb) list
1061 */
1062
1063 static obj_t lookup_in_frame(obj_t frame, obj_t symbol)
1064 {
1065 while(frame != obj_empty) {
1066 assert(TYPE(frame) == TYPE_PAIR);
1067 assert(TYPE(CAR(frame)) == TYPE_PAIR);
1068 assert(TYPE(CAAR(frame)) == TYPE_SYMBOL);
1069 if(CAAR(frame) == symbol)
1070 return CAR(frame);
(gdb) print frame->type.type
$2 = 13
The number 13 is the value TYPE_PAD. So instead of the expected
pair, frame points to a padding object.
You might guess at this point that the frame had not been fixed, and
since you know that the frame is referenced by the car of the
first pair in the environment, that’s the suspect reference. But in a
more complex situation this might not yet be clear. In such a
situation it can be useful to look at the sequence of events leading
up to the detection of the error. See Telemetry.
4.3. Example: allocating with wrong size¶
Here’s another kind of mistake: an off-by-one error in make_string
leading to the allocation of string objects with the wrong size:
static obj_t make_string(size_t length, char *string)
{
obj_t obj;
mps_addr_t addr;
size_t size = ALIGN_OBJ(offsetof(string_s, string) + length/* oops, forgot: +1 */);
do {
mps_res_t res = mps_reserve(&addr, obj_ap, size);
if (res != MPS_RES_OK) error("out of memory in make_string");
obj = addr;
obj->string.type = TYPE_STRING;
obj->string.length = length;
if (string) memcpy(obj->string.string, string, length+1);
else memset(obj->string.string, 0, length+1);
} while(!mps_commit(obj_ap, addr, size));
total += size;
return obj;
}
Here’s a test case that exercises this bug:
(define (church n f a) (if (eqv? n 0) a (church (- n 1) f (f a))))
(church 1000 (lambda (s) (string-append s "x")) "")
And here’s how it shows up in the debugger:
$ gdb ./scheme
GNU gdb 6.3.50-20050815 (Apple version gdb-1820) (Sat Jun 16 02:40:11 UTC 2012)
[...]
(gdb) run < test.scm
Starting program: example/scheme/scheme < test.scm
Reading symbols for shared libraries +............................. done
MPS Toy Scheme Example
[...]
9960, 0> church
Assertion failed: (0), function obj_skip, file scheme.c, line 2940.
10816, 0>
Program received signal SIGABRT, Aborted.
0x00007fff91aeed46 in __kill ()
(gdb) backtrace
#0 0x00007fff91aeed46 in __kill ()
#1 0x00007fff90509df0 in abort ()
#2 0x00007fff9050ae2a in __assert_rtn ()
#3 0x00000001000014e3 in obj_skip (base=0x1003f9b88) at scheme.c:2940
#4 0x0000000100068050 in amcScanNailedOnce (totalReturn=0x7fff5fbfef2c, moreReturn=0x7fff5fbfef28, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928, amc=0x1003fe278) at poolamc.c:1485
#5 0x0000000100067ca1 in amcScanNailed (totalReturn=0x7fff5fbff174, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928, amc=0x1003fe278) at poolamc.c:1522
#6 0x000000010006631f in AMCScan (totalReturn=0x7fff5fbff174, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928) at poolamc.c:1595
#7 0x000000010002686d in PoolScan (totalReturn=0x7fff5fbff174, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928) at pool.c:405
#8 0x0000000100074106 in traceScanSegRes (ts=1, rank=1, arena=0x10012a000, seg=0x1003fe928) at trace.c:1162
#9 0x000000010002b399 in traceScanSeg (ts=1, rank=1, arena=0x10012a000, seg=0x1003fe928) at trace.c:1222
#10 0x000000010002d020 in TraceQuantum (trace=0x10012a5a0) at trace.c:1833
#11 0x000000010001f2d2 in TracePoll (globals=0x10012a000) at trace.c:1981
#12 0x000000010000d75f in ArenaPoll (globals=0x10012a000) at global.c:684
#13 0x000000010000ea40 in mps_ap_fill (p_o=0x7fff5fbff3e0, mps_ap=0x1003fe820, size=208) at mpsi.c:961
#14 0x000000010000447d in make_string (length=190, string=0x0) at scheme.c:468
#15 0x0000000100008ca2 in entry_string_append (env=0x1003cbe38, op_env=0x1003cbe50, operator=0x1003fad48, operands=0x1003f9af8) at scheme.c:2562
#16 0x0000000100002fe4 in eval (env=0x1003cbe38, op_env=0x1003cbe50, exp=0x1003f9ae0) at scheme.c:1159
#17 0x0000000100005ff5 in entry_interpret (env=0x1003cb958, op_env=0x1003cb970, operator=0x1003f99d8, operands=0x1003f9948) at scheme.c:1340
#18 0x0000000100002fe4 in eval (env=0x1003cb958, op_env=0x1003cb970, exp=0x1003f9878) at scheme.c:1159
#19 0x000000010000206b in start (p=0x0, s=0) at scheme.c:3213
#20 0x000000010001287d in ProtTramp (resultReturn=0x7fff5fbff7a0, f=0x100001b80 <start>, p=0x0, s=0) at protix.c:132
#21 0x0000000100001947 in main (argc=1, argv=0x7fff5fbff808) at scheme.c:3314
(gdb) frame 3
#3 0x00000001000014e3 in obj_skip (base=0x1003f9b88) at scheme.c:2940
2940 assert(0);
(gdb) list
2935 break;
2936 case TYPE_PAD1:
2937 base = (char *)base + ALIGN_OBJ(sizeof(pad1_s));
2938 break;
2939 default:
2940 assert(0);
2941 fprintf(stderr, "Unexpected object on the heap\n");
2942 abort();
2943 return NULL;
2944 }
The object being skipped is corrupt:
(gdb) print obj->type.type
$1 = 4168560
What happened to it? It’s often helpful in these situations to have a look at nearby memory.
(gdb) x/20g obj
0x1003f9b88: 0x00000001003f9b70 0x00000001003fb000
0x1003f9b98: 0x0000000000000000 0x00000001003f9c90
0x1003f9ba8: 0x00000001003fb130 0x0000000000000000
0x1003f9bb8: 0x00000001003fb000 0x00000001003fb148
0x1003f9bc8: 0x0000000000000000 0x00000001003f9730
0x1003f9bd8: 0x00000001003f9a58 0x0000000000000000
0x1003f9be8: 0x00000001003f9bc8 0x00000001003fb000
0x1003f9bf8: 0x0000000000000000 0x00000001003fb0a0
0x1003f9c08: 0x00000001003f9b40 0x0000000000000004
0x1003f9c18: 0x000000010007b14a 0x0000000100005e30
You can see that this is a block containing mostly pairs (which have
tag 0 and consist of three words), though you can see an operator
(with tag 4) near the bottom. But what’s that at the start of the
block, where obj’s tag should be? It looks like a pointer. So
what’s in the memory just below obj? Let’s look at the previous
few words:
(gdb) x/10g (mps_word_t*)obj-8
0x1003f9b48: 0x00000001003f9ae0 0x00000001003fb000
0x1003f9b58: 0x0000000000000000 0x00000001003f9a80
0x1003f9b68: 0x00000001003f9b80 0x0000000000000005
0x1003f9b78: 0x0000000000000000 0x0000000000000000
0x1003f9b88: 0x00000001003f9b70 0x00000001003fb000
Yes: there’s a pair (with tag 0) at 0x1003f9b80. So it looks as
though the previous object was allocated with one size, but skipped
with a different size. The previous object being the string (with tag
5) at 0x1003f9b70 which has length 0 and so is three words long as
far as obj_skip is concerned:
(gdb) print obj_skip(0x1003f9b70)
$2 = (mps_addr_t) 0x1003f9b88
but the next object (the pair) was clearly allocated at
0x1003f9b80 (overwriting the last word of the string), so the
string must have been allocated with a size of only two words. This
should be enough evidence to track down the cause.
4.4. What next?¶
If you tracked down all your bugs, then the next step is the chapter Tuning the Memory Pool System for performance.
But if you’re still struggling, please contact us and see if we can help.
