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From: Thomas Rodgers <rodgertq@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r11-9547] libstdc++: Fix deadlock in atomic wait [PR104442] Date: Wed, 9 Feb 2022 20:32:29 +0000 (GMT) [thread overview] Message-ID: <20220209203229.7585C3858433@sourceware.org> (raw) https://gcc.gnu.org/g:5669a60e2fcc90afaa16fe20391cdcf18ab398d7 commit r11-9547-g5669a60e2fcc90afaa16fe20391cdcf18ab398d7 Author: Thomas Rodgers <rodgert@appliantology.com> Date: Wed Feb 9 12:29:19 2022 -0800 libstdc++: Fix deadlock in atomic wait [PR104442] This issue was observed as a deadlock in 29_atomics/atomic/wait_notify/100334.cc on vxworks. When a wait is "laundered" (e.g. type T* does not suffice as a waitable address for the platform's native waiting primitive), the address waited is that of the _M_ver member of __waiter_pool_base, so several threads may wait on the same address for unrelated atomic<T> objects. As noted in the PR, the implementation correctly exits the wait for the thread whose data changed, but not for any other threads waiting on the same address. As noted in the PR the __waiter::_M_do_wait_v member was correctly exiting but the other waiters were not reloading the value of _M_ver before re-entering the wait. Moving the spin call inside the loop accomplishes this, and is consistent with the predicate accepting version of __waiter::_M_do_wait. libstdc++-v3/ChangeLog: PR libstdc++/104442 * include/bits/atomic_wait.h (__waiter::_M_do_wait_v): Move spin loop inside do loop so that threads failing the wait, reload _M_ver. (cherry picked from commit 4cf3c339815cdfa636b25a512f91b63d7c313fd6) Diff: --- libstdc++-v3/include/bits/atomic_wait.h | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h index f6a1c0d8c32..d7ded906360 100644 --- a/libstdc++-v3/include/bits/atomic_wait.h +++ b/libstdc++-v3/include/bits/atomic_wait.h @@ -390,12 +390,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION void _M_do_wait_v(_Tp __old, _ValFn __vfn) { - __platform_wait_t __val; - if (__base_type::_M_do_spin_v(__old, __vfn, __val)) - return; - do { + __platform_wait_t __val; + if (__base_type::_M_do_spin_v(__old, __vfn, __val)) + return; __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val); } while (__detail::__atomic_compare(__old, __vfn()));
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