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From: "lewissbaker.opensource at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99537] New: Wrong memory_order used in stop_token
 ref-counting
Date: Thu, 11 Mar 2021 02:00:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99537

            Bug ID: 99537
           Summary: Wrong memory_order used in stop_token ref-counting
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lewissbaker.opensource at gmail dot com
  Target Milestone: ---

In the implementation of stop_token the _Stop_state_t implements
reference-counting for tracking shared ownership of the stop-state.

This is done via two methods:

      void
      _M_add_owner() noexcept
      {
        _M_owners.fetch_add(1, memory_order::relaxed);
      }

      void
      _M_release_ownership() noexcept
      {
        if (_M_owners.fetch_sub(1, memory_order::release) =3D=3D 1)
          delete this;
      }

Other than initialising the _M_owners atomic value to 1, these are the only=
 two
accesses of the _M_owners variable.

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 memor=
y.=