| Message ID | CACT4Y+b5rPevB7foQqmvu+PyZP=wSQ+vCWY7YbKgj3OoUFKmgA@mail.gmail.com |
|---|---|
| State | New |
| Headers | show |
On 01/09/15 17:42 +0200, Dmitry Vyukov wrote: >On Tue, Sep 1, 2015 at 5:08 PM, Jonathan Wakely <jwakely@redhat.com> wrote: >> On 01/09/15 16:56 +0200, Dmitry Vyukov wrote: >>> >>> I don't understand how a new gcc may not support __atomic builtins on >>> ints. How it is even possible? That's a portable API provided by >>> recent gcc's... >> >> >> The built-in function is always defined, but it might expand to a call >> to an external function in libatomic, and it would be a regression for >> code using std::string to start requiring libatomic (although maybe it >> would be necessary if it's the only way to make the code correct). >> >> I don't know if there are any targets that define __GTHREADS and also >> don't support __atomic_load(int*, ...) without libatomic. If such >> targets exist then adding a new configure check that only depends on >> __atomic_load(int*, ...) would mean we keep supporting those targets. >> >> Another option would be to simply do: >> >> bool >> _M_is_shared() const _GLIBCXX_NOEXCEPT >> #if defined(__GTHREADS) >> + { return __atomic_load(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0; } >> +#else >> { return this->_M_refcount > 0; } >> +#endif >> >> and see if anyone complains! > >I like this option! >If a platform uses multithreading and has non-inlined atomic loads, >then the way to fix this is to provide inlined atomic loads rather >than to fix all call sites. > >Attaching new patch. Please take another look. This looks good. Torvald suggested that it would be useful to add a similar comment to the release operation in _M_dispose, so that both sides of the release-acquire are similarly documented. Could you add that and provide a suitable ChangeLog entry? Thanks! >Index: include/bits/basic_string.h >=================================================================== >--- include/bits/basic_string.h (revision 227363) >+++ include/bits/basic_string.h (working copy) >@@ -2601,11 +2601,32 @@ > > 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
Index: include/bits/basic_string.h =================================================================== --- include/bits/basic_string.h (revision 227363) +++ include/bits/basic_string.h (working copy) @@ -2601,11 +2601,32 @@ 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