http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52839
--- Comment #19 from Alan Modra <amodra at gmail dot com> 2012-04-10 15:13:24
UTC ---
I think I was on the right track when I questioned whether the problem might be
mixing atomics and mutex protected ops, but was looking in the wrong place. I
should have looked at default_weaktoshared.o rather than libstdc++.so.
objdump -drS default_weaktoshared.o shows
[snip]
void* thread_hammer(void* opaque_weak)
{
0: 94 21 ff d0 stwu r1,-48(r1)
static inline _Atomic_word
__attribute__ ((__unused__))
__exchange_and_add_dispatch(_Atomic_word* __mem, int __val)
{
#ifdef __GTHREADS
if (__gthread_active_p())
4: 3d 20 00 00 lis r9,0
6: R_PPC_ADDR16_HA pthread_cancel
8: 7d 80 00 26 mfcr r12
c: 7c 08 02 a6 mflr r0
10: 39 29 00 00 addi r9,r9,0
12: R_PPC_ADDR16_LO pthread_cancel
14: 2e 09 00 00 cmpwi cr4,r9,0
18: 93 c1 00 28 stw r30,40(r1)
typedef typename __traits_type::pointer pointer;
_GLIBCXX_CONSTEXPR __normal_iterator() : _M_current(_Iterator()) { }
explicit
__normal_iterator(const _Iterator& __i) : _M_current(__i) { }
1c: 3f c0 00 01 lis r30,1
20: 90 01 00 34 stw r0,52(r1)
24: 93 81 00 20 stw r28,32(r1)
28: 93 a1 00 24 stw r29,36(r1)
2c: 93 e1 00 2c stw r31,44(r1)
30: 91 81 00 1c stw r12,28(r1)
34: 7c 7c 1b 78 mr r28,r3
38: 63 de 86 a0 ori r30,r30,34464
3c: 83 a3 00 00 lwz r29,0(r3)
40: 48 00 00 18 b 58 <_Z13thread_hammerPv+0x58>
44: 60 00 00 00 nop
48: 60 00 00 00 nop
4c: 60 00 00 00 nop
0x9908b0dful, 11, 7,
0x9d2c5680ul, 15,
0xefc60000ul, 18> rng;
wp_vector_t::iterator cur_weak = weak_pool.begin();
for (unsigned int i = 0; i < HAMMER_REPEAT; ++i)
50: 37 de ff ff addic. r30,r30,-1
54: 41 82 00 ac beq- 100 <_Z13thread_hammerPv+0x100>
// now that __weak_count is defined we can define this constructor:
template<_Lock_policy _Lp>
inline
__shared_count<_Lp>::
__shared_count(const __weak_count<_Lp>& __r)
: _M_pi(__r._M_pi)
58: 83 fd 00 04 lwz r31,4(r29)
{
if (_M_pi != 0)
5c: 2f 9f 00 00 cmpwi cr7,r31,0
60: 41 9e 00 cc beq- cr7,12c <_Z13thread_hammerPv+0x12c>
inline void
_Sp_counted_base<_S_atomic>::
_M_add_ref_lock()
{
// Perform lock-free add-if-not-zero operation.
_Atomic_word __count = _M_use_count;
64: 81 3f 00 04 lwz r9,4(r31)
68: 91 21 00 08 stw r9,8(r1)
do
{
if (__count == 0)
6c: 2f 89 00 00 cmpwi cr7,r9,0
70: 41 9e 00 d8 beq- cr7,148 <_Z13thread_hammerPv+0x148>
// Replace the current counter value with the old value + 1, as
// long as it's not changed meanwhile.
}
while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
true, __ATOMIC_ACQ_REL,
__ATOMIC_RELAXED));
74: 81 01 00 08 lwz r8,8(r1)
78: 39 29 00 01 addi r9,r9,1
7c: 38 7f 00 04 addi r3,r31,4
80: 7c 20 04 ac lwsync
84: 7d 40 18 28 lwarx r10,0,r3
88: 7c 0a 40 00 cmpw r10,r8
8c: 40 82 00 0c bne- 98 <_Z13thread_hammerPv+0x98>
90: 7d 20 19 2d stwcx. r9,0,r3
94: 4c 00 01 2c isync
98: 91 41 00 08 stw r10,8(r1)
_Sp_counted_base<_S_atomic>::
_M_add_ref_lock()
{
// Perform lock-free add-if-not-zero operation.
_Atomic_word __count = _M_use_count;
do
9c: 40 82 00 a4 bne- 140 <_Z13thread_hammerPv+0x140>
a0: 41 92 00 ac beq- cr4,14c <_Z13thread_hammerPv+0x14c>
return __exchange_and_add(__mem, __val);
a4: 38 80 ff ff li r4,-1
a8: 48 00 00 01 bl a8 <_Z13thread_hammerPv+0xa8>
a8: R_PPC_REL24 _ZN9__gnu_cxx18__exchange_and_addEPVii
void
_M_release() // nothrow
{
// Be race-detector-friendly. For more info see bits/c++config.
_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
ac: 2f 83 00 01 cmpwi cr7,r3,1
b0: 40 9e ff a0 bne+ cr7,50 <_Z13thread_hammerPv+0x50>
{
_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
_M_dispose();
b4: 81 3f 00 00 lwz r9,0(r31)
b8: 7f e3 fb 78 mr r3,r31
bc: 81 29 00 08 lwz r9,8(r9)
c0: 7d 29 03 a6 mtctr r9
c4: 4e 80 04 21 bctrl
_GLIBCXX_WRITE_MEM_BARRIER;
That certainly looks like _M_use_count is fiddled with both by atomics and
__exchange_and_add with pthread mutex.
