Defect Report #407

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Version: 1.4
Date: April 2014
Subject:

Summary

For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that Ais sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M.

For atomic modifications A and B of an atomic object M, if there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes B in S, then B occurs later than A in the modification order of M.

For atomic modifications A and B of an atomic object M, if there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes Y in S, then B occurs later than A in the modification order of M.

Suggested Technical Corrigendum

Committee Discussion

• The changes are difficult to fully understand - a diagram might help.
• A paper for the next meeting would help the Committee make progress on this.
• Some concern about having C and C++ differ.

Committee Discussion

• No work was done on this defect between meetings, so no additional information has been provided.
• There seems to be two issues here, and this should possibility be two defect reports rather than one.
• No known liaison has taken place with WG 21 on this issue.
• The proposed words are inappropriate for C, still need words that work for C.
• There seems to be editorial issues with the proposed words for C++ as well.
• There is consensus to adopt something along the lines of the first proposed change, with wording that is consistent with C.
• WG 14 needs to stay in step with WG 21.
• Clark Nelson took an action item to liaise with WG 21.

Committee Discussion

• This item has also become WG21 Library Issue 2130.
• Rajan presented a diagram and argued that we should adopt the first part and that the second two parts were redundant.

• After 7.17.3 paragraph 11 add the following:

  For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M.

Committee Discussion

• The corresponding words in proposed working draft for C++ have changed, and the proposal above is no longer appropriate.
• The corresponding paragraph reference for C is still 7.13.3 paragraph 11.
• The following change has been applied to the C++ working draft:

  Change 29.3 [atomics.order] paragraph 7 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]

  -7- For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M. For atomic modifications A and B of an atomic object M, B occurs later than A in the modification order of M if:

  • there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes B in S, or
  • there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes Y in S, or
  • there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S.

Committee Discussion

WG21 liaison has been asked to ascertain status of this w.r.t. C++14 and to provide a suggested TC.

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