https://gcc.gnu.org/g:97ea9d7b021c3e9f8d0c7e371d40ded811f76dac
commit r16-989-g97ea9d7b021c3e9f8d0c7e371d40ded811f76dac
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Thu Jan 9 22:32:02 2025 +0000
libstdc++: Fix race condition in new atomic notify code
When using a proxy object for atomic waiting and notifying operations,
we need to ensure that the _M_ver value is always incremented by a
notifying operation, even if we return early without doing the futex
wake syscall. Otherwise we get missed wake-ups because the notifying
thread doesn't modify the value that other threads are doing a futex
wait on.
libstdc++-v3/ChangeLog:
* include/bits/atomic_wait.h (__notify_impl): Increment the
proxy value before returning early for the uncontended case.
Diff:
---
libstdc++-v3/include/bits/atomic_wait.h | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
diff --git a/libstdc++-v3/include/bits/atomic_wait.h
b/libstdc++-v3/include/bits/atomic_wait.h
index 172910a4e34f..725ea030245d 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -406,18 +406,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
__waiter_pool_impl* __pool = nullptr;
- if (__args & __wait_flags::__track_contention)
- {
- __pool = &__waiter_pool_impl::_S_impl_for(__addr);
- if (!__pool->_M_waiting())
- return;
- }
-
const __platform_wait_t* __wait_addr;
if (__args & __wait_flags::__proxy_wait)
{
- if (!__pool)
- __pool = &__waiter_pool_impl::_S_impl_for(__addr);
+ __pool = &__waiter_pool_impl::_S_impl_for(__addr);
// Waiting for *__addr is actually done on the proxy's _M_ver.
__wait_addr = &__pool->_M_ver;
__atomic_fetch_add(&__pool->_M_ver, 1, __ATOMIC_RELAXED);
@@ -430,6 +422,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else // Use the atomic variable's own address.
__wait_addr = __addr;
+ if (__args & __wait_flags::__track_contention)
+ {
+ if (!__pool)
+ __pool = &__waiter_pool_impl::_S_impl_for(__addr);
+ if (!__pool->_M_waiting())
+ return;
+ }
+
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
__platform_notify(__wait_addr, __all);
#else
