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libstdc++: fix data races in basic_string implementation 

* include/bits/basic_string.h: Fix data races on _M_refcount.

From-SVN: r227403
This commit is contained in:
Dmitry Vyukov 2015-09-02 07:35:20 -07:00 committed by Dmitry Vyukov
parent 7e4713f887
commit 710465235b
2 changed files with 35 additions and 2 deletions
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4
libstdc++-v3/ChangeLog
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@ -1,3 +1,7 @@
2015-09-02 Dmitry Vyukov <dvyukov@google.com>
* include/bits/basic_string.h: Fix data races on _M_refcount.
2015-09-02 Sebastian Huber <sebastian.huber@embedded-brains.de>
PR libstdc++/67408

33
libstdc++-v3/include/bits/basic_string.h
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@ -2601,11 +2601,32 @@ _GLIBCXX_END_NAMESPACE_CXX11
bool
_M_is_leaked() const _GLIBCXX_NOEXCEPT
{ return this->_M_refcount < 0; }
{
#if defined(__GTHREADS)
// _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
// so we need to use an atomic load. However, _M_is_leaked
// predicate does not change concurrently (i.e. the string is either
// leaked or not), so a relaxed load is enough.
return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0;
#else
return this->_M_refcount < 0;
#endif
}
bool
_M_is_shared() const _GLIBCXX_NOEXCEPT
{ return this->_M_refcount > 0; }
{
#if defined(__GTHREADS)
// _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
// so we need to use an atomic load. Another thread can drop last
// but one reference concurrently with this check, so we need this
// load to be acquire to synchronize with release fetch_and_add in
// _M_dispose.
return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0;
#else
return this->_M_refcount > 0;
#endif
}
void
_M_set_leaked() _GLIBCXX_NOEXCEPT
@ -2654,6 +2675,14 @@ _GLIBCXX_END_NAMESPACE_CXX11
{
// Be race-detector-friendly. For more info see bits/c++config.
_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&this->_M_refcount);
// Decrement of _M_refcount is acq_rel, because:
// - all but last decrements need to release to synchronize with
// the last decrement that will delete the object.
// - the last decrement needs to acquire to synchronize with
// all the previous decrements.
// - last but one decrement needs to release to synchronize with
// the acquire load in _M_is_shared that will conclude that
// the object is not shared anymore.
if (__gnu_cxx::__exchange_and_add_dispatch(&this->_M_refcount,
-1) <= 0)
{