A failed assertion was observed with std::atomic<bool>::wait when the
loop in __atomic_wait_address is entered and calls _M_setup_wait a
second time. When the first call to _M_setup_wait makes a call to
_M_setup_proxy_wait it decides that a proxy wait is needed for bool, and
updates the _M_obj and _M_obj_size members to refer to the futex in the
proxy state, instead of referring to the bool object. The next time
_M_setup_wait is called it calls _M_setup_proxy_wait again but now it
sees _M_obj_size == sizeof(futex) and so this time decides a proxy wait
is *not* needed, and then fails the __glibcxx_assert(_M_obj == addr)
check.
The problem is that _M_setup_wait is calling _M_setup_proxy_wait twice,
when it should avoid the second call because the decision to use a proxy
wait has already been made. The caller should avoid making the second
call by checking whether _M_obj != addr, because that implies that a
decision to make a proxy wait has already happened.
This change fixes the bug, by preventing a second call to
_M_setup_proxy_wait after we've changed _M_obj and _M_obj_size. But it
doesn't prevent a second call in the case where _M_setup_proxy_wait
returned false because we made a runtime decision to do a non-proxy
wait. That is suboptimal, because it means that code which can wait on
the atomic variable directly makes redundant calls into the library just
to be told the same answer every time. That's not a problem for now
because firstly, it's only a performance degradation not a correctness
bug, and secondly because we currently have no targets that ever make a
runtime decision to do a non-proxy wait. Linux makes a compile-time
decision to do non-proxy waits for futex-sized objects and does proxy
waits for everything else, and all non-Linux targets always do proxy
waits. So solving the performance problem for non-proxy waits can happen
later. We might want to set a bit in __wait_args::_M_flags to say "I
already told you this isn't a proxy wait, stop asking me".
libstdc++-v3/ChangeLog:
* include/bits/atomic_wait.h (__wait_args::_M_setup_wait): Do
not call _M_setup_proxy_wait again if a proxy wait was already
set up.
---
Tested x86_64-linux and x86_64-freebsd14 (just the parts that use atomic
waits).
libstdc++-v3/include/bits/atomic_wait.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/bits/atomic_wait.h
b/libstdc++-v3/include/bits/atomic_wait.h
index b6240a95370f..a677dbf0a040 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -230,7 +230,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
if constexpr (!__platform_wait_uses_type<_Tp>)
- if (_M_setup_proxy_wait(__addr))
+ if (_M_obj != __addr || _M_setup_proxy_wait(__addr))
{
// We will use a proxy wait for this object.
// The library has set _M_obj and _M_obj_size and _M_old.
--
2.52.0
