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4475. The description to single total order in [thread.mutex.requirements.mutex.general] is hollow

Section: 32.6.4.2.1 [thread.mutex.requirements.mutex.general] Status: New Submitter: jim x Opened: 2025-11-14 Last modified: 2025-11-16

Priority: Not Prioritized

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Discussion:

32.6.4.2.1 [thread.mutex.requirements.mutex.general] p4 says:

For purposes of determining the existence of a data race, these behave as atomic operations (6.10.2 [intro.multithread]). The lock and unlock operations on a single mutex appears to occur in a single total order.

Even for atomic operations, the precondition for ordering them in a single total order is that they must be memory_order::seq_cst operations, such that we can form the total order to reason.

Put aside the fact that we impose the preconditions on unlock and lock. Is this a possible total order if lock reads unlock_1, but there is a unlock_2 between them

unlock_1 < unlock_2 < lock

First, although we have said that lock and unlock operations behave as atomic operations, and lock reads unlock_1, meaning that unlock_1 is coherence-ordered before lock, however, we don't specify that they are memory_order::seq_cst operations, so 32.5.4 [atomics.order] p4 doesn't apply here

Second, for every pair of atomic operations A and B on an object M, where A is coherence-ordered before B, the following four conditions are required to be satisfied by S:

• (4.1) — if A and B are both memory_order::seq_cst operations, then A precedes B in S; and

So, it is not helpful to decide that unlock_1 precedes lock in a single total order. Similarly, excluding unlock_1 < unlock_2 < lock is not possible.

Suggested resolution:

The lock and unlock operations on a single mutex appears to occur in a single total order; for this purpose, these operations are considered as memory_order::seq_cst operations

Proposed resolution:

This wording is relative to N5014.

1. Modify 32.6.4.2.1 [thread.mutex.requirements.mutex.general] as indicated:

   -4- The implementation provides lock and unlock operations, as described below. For purposes of determining the existence of a data race, these behave as atomic operations (6.10.2 [intro.multithread]). The lock and unlock operations on a single mutex appears to occur in a single total order; for this purpose, these operations are considered as memory_order::seq_cst operations.