[gcc r10-9594] libstdc++: Use acq_rel memory ordering [PR 99537]

[Jonathan Wakely]

https://gcc.gnu.org/g:ad9ae82ed1c33e1803f33bb367b0b4c9ee2e3e63

commit r10-9594-gad9ae82ed1c33e1803f33bb367b0b4c9ee2e3e63
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 11 16:44:57 2021 +0000

    libstdc++: Use acq_rel memory ordering [PR 99537]
    
    As Lewis Baker wrote in the PR:
    
    > The 'fetch_sub()' operation in _M_release_ownership() should be using
    > memory_order::acq_rel instead of memory_order::release. The use of
    > 'release' only is insufficient as it does not synchronise with any
    > corresponding 'acquire' operation.
    
    > With the current implementation, it's possible that a prior write to
    > one of the _M_value or _M_head data-members by a thread releasing the
    > second-to-last reference might not be visible to another thread that
    > releases the last reference and frees the memory, resulting in
    > potential write to freed memory.
    
    This simply changes the memory order to acq_rel as suggested.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/99537
            * include/std/stop_token (_Stop_state_t::_M_release_ownership):
            Use acq_rel memory ordering.
    
    (cherry picked from commit 15825b17cf3fbf28181c51fe94a2898f448f915c)

Diff:
---
 libstdc++-v3/include/std/stop_token | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/stop_token b/libstdc++-v3/include/std/stop_token
index 80f50ea83ca..34b613e7c1a 100644
--- a/libstdc++-v3/include/std/stop_token
+++ b/libstdc++-v3/include/std/stop_token
@@ -191,7 +191,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void
       _M_release_ownership() noexcept
       {
-	if (_M_owners.fetch_sub(1, memory_order::release) == 1)
+	if (_M_owners.fetch_sub(1, memory_order::acq_rel) == 1)
 	  delete this;
       }