Issue 0407: SC fences do not restrict modification order enough

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: WG 14, Batty
Date: 2011-10-14
Reference document: N1584
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm

Summary

C11 seems to omit the restriction imposed in C++11 in 29.3p7 (from N3291):

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.

Furthermore, it seems that both C11 and C++11 are missing the following two derivatives of this rule:

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

See above.

Comment from WG14 on 2017-11-03:

Oct 2011 meeting

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.

Feb 2012 meeting

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.

Oct 2012 meeting

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.

Oct 2013 meeting

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.

Apr 2014 meeting

Committee Discussion

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

Oct 2014 meeting

Committee Discussion

A paper N1856 was provided that discusses the drift between the two Standards and a first cut at some possible wording changes, as follows. It was not, however, discussed, but does provide insight as to the necessary direction for a resolution to this DR.

1. [Drafting note: The project editor is kindly asked to consider to replace in 1.10 [intro.multithread] p17 the phrase "before an operation B on M" by "before a modification B of M".]

2. Change 7.17.3 paragraph 11 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]

   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.

   [ Note: memory_order_seq_cst ensures sequential consistency only for a program that is free of data races and uses exclusively memory_order_seq_cst operations. Any use of weaker ordering will invalidate this guarantee unless extreme care is used. In particular, memory_order_seq_cst fences ensure a total order only for the fences themselves. Fences cannot, in general, be used to restore sequential consistency for atomic operations with weaker ordering specifications. —end note ]

Apr 2015 meeting

Committee Discussion

The provided words were adopted as the Proposed Technical Corrigendum. The project editor is asked to review and replace the phrase "before an operation B on M" by "before a modification B of M".

Proposed Technical Corrigendum

Change 7.17.3 paragraph 11 from:

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.

to:

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.