[gcc r11-9547] libstdc++: Fix deadlock in atomic wait [PR104442]

[Thomas Rodgers]

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()));