Issue 3756: Is the std::atomic_flag class signal-safe? (original) (raw)

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3756. Is the std::atomic_flag class signal-safe?

Section: 17.14.5 [support.signal], 32.5.10 [atomics.flag] Status: C++23 Submitter: Ruslan Baratov Opened: 2022-08-18 Last modified: 2023-11-22

Priority: 3

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Discussion:

Following document number N4910 about signal-safe instructions 17.14.5 [support.signal] Signal handlers, and it's unclear whetherstd::atomic_flag is signal-safe.

Formally it doesn't fit any of the mentioned conditions:

However, std::atomic_flag seem to fit well here, it's atomic, and it's always lock-free.

The suggestion is as follows: If std::atomic_flag is signal-safe, then it should be explicitly mentioned in 17.14.5 [support.signal], e.g., the following lines should be added:

If the std::atomic_flag is not signal-safe, the following note could be added:

[Note: Even though atomic_flag is atomic and lock-free, it's not signal-safe. — _end note_]

[2022-09-23; Reflector poll]

Set priority to 3 after reflector poll. Send to SG1.

Another way to fix this is to add is_always_lock_free (=true) andis_lock_free() { return true; } to atomic_flag.

[Kona 2022-11-10; SG1 yields a recommendation]

Poll: Adopt the proposed resolution for LWG3756
"f is a non-static member function invoked on an atomic_flag object, or"
"f is a non-member function, and every pointer-to- atomic argument passed to f is atomic_flag, or"

SF F N A SA 11 3 0 0 0

Unanimous consent

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

  1. Modify 17.14.5 [support.signal] as indicated:

    -1- A call to the function signal synchronizes with any resulting invocation of the signal handler so installed.

    -2- A plain lock-free atomic operation is an invocation of a functionf from 32.5 [atomics], such that:

    1. (2.1) —f is the function atomic_is_lock_free(), or
    2. (2.2) —f is the member function is_lock_free(), or
    3. (2.?) —f is a non-static member function invoked on anatomic_flag object, or
    4. (2.?) —f is a non-member function, and every pointer-to-atomic argument passed to f is atomic_flag, or
    5. (2.3) —f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or
    6. (2.4) —f is a non-member function, and for every pointer-to-atomic argumentA passed to f,atomic_is_lock_free(A) yields true.

    -3- An evaluation is signal-safe unless it includes one of the following:

    1. (3.1) — a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;
      [Note 1: This implicitly excludes the use of new anddelete expressions that rely on a library-provided memory allocator. — _end note_]
    2. (3.2) — an access to an object with thread storage duration;
    3. (3.3) — a dynamic_cast expression;
    4. (3.4) — throwing of an exception;
    5. (3.5) — control entering a try-block or function-try-block;
    6. (3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.10.3.3 [basic.start.dynamic], 8.10 [stmt.dcl])206; or
    7. (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.10 [stmt.dcl]).

    A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.

[2022-11-11; Jonathan provides improved wording]

[Kona 2022-11-11; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

  1. Modify 17.14.5 [support.signal] as indicated:

    -1- A call to the function signal synchronizes with any resulting invocation of the signal handler so installed.

    -2- A plain lock-free atomic operation is an invocation of a functionf from 32.5 [atomics], such that:

    1. (2.1) —f is the function atomic_is_lock_free(), or
    2. (2.2) —f is the member function is_lock_free(), or
    3. (2.?) —f is a non-static member function of class atomic_flag, or
    4. (2.?) —f is a non-member function, and the first parameter of fhas type cv atomic_flag*, or
    5. (2.3) —f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or
    6. (2.4) —f is a non-member function, and for every pointer-to-atomic argumentA passed to f,atomic_is_lock_free(A) yields true.

    -3- An evaluation is signal-safe unless it includes one of the following:

    1. (3.1) — a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;
      [Note 1: This implicitly excludes the use of new anddelete expressions that rely on a library-provided memory allocator. — _end note_]
    2. (3.2) — an access to an object with thread storage duration;
    3. (3.3) — a dynamic_cast expression;
    4. (3.4) — throwing of an exception;
    5. (3.5) — control entering a try-block or function-try-block;
    6. (3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.10.3.3 [basic.start.dynamic], 8.10 [stmt.dcl])206; or
    7. (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.10 [stmt.dcl]).

    A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.