[PATCH] libstdc++: Fix deadlock in atomic wait [PR104442]

[Thomas Rodgers <trodgers@redhat.com>]

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This issue was observed as a deadloack 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>'s. As noted in the PR, the
implementation correctly exits the wait for the thread who's 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 reloaded 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.

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From ee66736beca3dce4bc09350c5407a2ac7219fbec Mon Sep 17 00:00:00 2001
From: Thomas Rodgers <rodgert@appliantology.com>
Date: Tue, 8 Feb 2022 16:33:36 -0800
Subject: [PATCH] libstdc++: Fix deadlock in atomic wait [PR104442]

This issue was observed as a deadloack 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>'s. As noted in the PR, the
implementation correctly exits the wait for the thread who's 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 reloaded 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.
---
 libstdc++-v3/include/bits/atomic_wait.h | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index d7de0d7eb9e..33ce26ade1b 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -332,7 +332,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else
 	      {
-		__atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+		__atomic_load(__addr, &__val, __ATOMIC_SEQ_CST);
 	      }
 	    return __atomic_spin(__pred, __spin);
 	  }
@@ -388,12 +388,11 @@ _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()));
-- 
2.34.1