Issue 0405: mutex specification not aligned with C++11 on total order

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

The C11 specification of mutexes is missing the total order over all the calls on a particular mutex. This is present in C++11. The following is from 30.4.1.2p5 in N3291:

For purposes of determining the existence of a data race, these behave as atomic operations (1.10). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [ Note: this can be viewed as the modification order (1.10) of the mutex. — end note ]

The synchronisation in 7.26.4 is defined in terms of some order over these calls, even though none is specified, for instance 7.26.4.4p2 reads:

Prior calls to mtx_unlock on the same mutex shall synchronize with this operation.

This seems like simple omission. We suggest adding a new paragraph to 7.26.4 that matches C++11:

For purposes of determining the existence of a data race, mutex calls behave as atomic operations. The lock and unlock operations on a single mutex shall appear to occur in a single total order.

NOTE: This can be viewed as the modification order of the mutex.

Suggested Technical Corrigendum

See above.

Comment from WG14 on 2017-11-03:

Oct 2011 meeting

Committee discussion

• The quoted text was added to C++11 after WG 14 voted out the FDIS in London.
• The consensus was that this is probably an oversight, and should be changed along the lines that are recommended above.

Feb 2012 meeting

Committee Discussion

• The wording in the proposed wording is not correct, should read "shall occur" rather than "shall appear to occur."
• No consensus reached on the words, Clark Nelson took action item to furnish words for the next meeting.

Oct 2012 meeting

Committee Discussion

Add the following as 7.26.4 p1 and p2:

For purposes of determining the existence of a data race, lock and unlock operations behave as atomic operations. All lock and unlock operations on a particular mutex occur in some particular total order.

NOTE: This total order can be viewed as the modification order of the mutex.

Apr 2013 meeting

Committee Discussion

Accept wording from Oct 2012 as proposed technical corrigendum

Proposed Technical Corrigendum

Add the following as 7.26.4 p1 and p2:

For purposes of determining the existence of a data race, lock and unlock operations behave as atomic operations. All lock and unlock operations on a particular mutex occur in some particular total order.

NOTE: This total order can be viewed as the modification order of the mutex.