Actually, for number of threads >=3 this patch is better:
--- old/libstdc++-v3/include/bits/basic_string.h 2012-05-30 17:03:15.974035000 -0400
+++ new/libstdc++-v3/include/bits/basic_string.h 2012-05-31 13:06:39.903442000 -0400
@@ -224,14 +224,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_refdata() throw()
{ return reinterpret_cast<_CharT*>(this + 1); }
_CharT*
_M_grab(const _Alloc& __alloc1, const _Alloc& __alloc2)
{
- return (!_M_is_leaked() && __alloc1 == __alloc2)
- ? _M_refcopy() : _M_clone(__alloc1);
+ _CharT* res = (_CharT*)0;
+ if(!_M_is_leaked() && __alloc1 == __alloc2) {
+ res = _M_refcopy();
+ }
+ if (res == (_CharT*)0) {
+ res = _M_clone(__alloc1);
+ }
+ return res;
}
// Create & Destroy
static _Rep*
_S_create(size_type, size_type, const _Alloc&);
@@ -259,13 +265,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_CharT*
_M_refcopy() throw()
{
#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
if (__builtin_expect(this != &_S_empty_rep(), false))
#endif
- __gnu_cxx::__atomic_add_dispatch(&this->_M_refcount, 1);
+ {
+ // Here we try to detect and compensate an unexpected decrement of
+ // _M_refcount from another thread. In the properly designed code
+ // this should not happen ever! If we do nothing about this
+ // condition then the string is going to be freed twice.
+ // Unfortunatelly, some cases that lead to this condition are not
+ // obvious for the developers, e.g. non-constant
+ // methods operator [], at(), begin, end, etc.
+ // In many of these cases the string is not going to be
+ // changed from another thread and it's actually safe to
+ // clone the string.
+ if (__gnu_cxx::__exchange_and_add_dispatch(&this->_M_refcount, 1) < 0) {
+ __gnu_cxx::__atomic_add_dispatch(&this->_M_refcount, -1);
+ return (_CharT*)0;
+ }
+ }
return _M_refdata();
} // XXX MT
_CharT*
_M_clone(const _Alloc&, size_type __res = 0);
};
