From: Thomas Rodgers <rodgert@appliantology.com>
Date: Wed, 9 Feb 2022 20:29:19 +0000 (-0800)
Subject: libstdc++: Fix deadlock in atomic wait [PR104442]
X-Git-Tag: basepoints/gcc-13~1162
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4cf3c339815cdfa636b25a512f91b63d7c313fd6;p=thirdparty%2Fgcc.git

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.
---

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index d7de0d7eb9e6..6ce7f9343cf5 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -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()));