* [committed] libstdc++: Use acq_rel memory ordering [PR 99537]
@ 2021-03-11 17:54 Jonathan Wakely
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From: Jonathan Wakely @ 2021-03-11 17:54 UTC (permalink / raw)
To: libstdc++, gcc-patches
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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.
Tested powerpc64le-linux. Committed to trunk.
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commit 15825b17cf3fbf28181c51fe94a2898f448f915c
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Thu Mar 11 16:44:57 2021
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.
diff --git a/libstdc++-v3/include/std/stop_token b/libstdc++-v3/include/std/stop_token
index 83905f6525f..fffc215d2a8 100644
--- a/libstdc++-v3/include/std/stop_token
+++ b/libstdc++-v3/include/std/stop_token
@@ -185,7 +185,7 @@ _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;
}
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2021-03-11 17:54 [committed] libstdc++: Use acq_rel memory ordering [PR 99537] Jonathan Wakely
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