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

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of WP status.

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

Section: 17.13.5 [support.signal], 33.5.10 [atomics.flag] Status: WP Submitter: Ruslan Baratov Opened: 2022-08-18 Last modified: 2023-02-13

Priority: 3

View all issues with WP status.

Discussion:

Following document number N4910 about signal-safe instructions 17.13.5 [support.signal] Signal handlers, and it's unclear whether std::atomic_flag is signal-safe.

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

f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or

(there is no is_lock_free method in std::atomic_flag class)
f is a non-member function, and for every pointer-to-atomic argument A passed to f, atomic_is_lock_free(A) yields true

(std::atomic_flag object can't be passed to atomic_is_lock_free as argument)

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 17.13.5 [support.signal], e.g., the following lines should be added:

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

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) and is_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Previous resolution [SUPERSEDED]: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.

Modify 17.13.5 [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 function f from 33.5 [atomics], such that:

(2.1) — f is the function atomic_is_lock_free(), or

(2.2) — f is the member function is_lock_free(), or

(2.?) — f is a non-static member function invoked on an atomic_flag object, or

(2.?) — f is a non-member function, and every pointer-to-atomic argument passed to f is atomic_flag, or

(2.3) — f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or

(2.4) — f is a non-member function, and for every pointer-to-atomic argument A passed to f, atomic_is_lock_free(A) yields true.

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

(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 and delete expressions that rely on a library-provided memory allocator. — end note]

(3.2) — an access to an object with thread storage duration;

(3.3) — a dynamic_cast expression;

(3.4) — throwing of an exception;

(3.5) — control entering a try-block or function-try-block;

(3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.9.3.3 [basic.start.dynamic], 8.8 [stmt.dcl])²⁰⁶; or

(3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.8 [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 Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

Modify 17.13.5 [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 function f from 33.5 [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 f has 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 argument A 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 and delete 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 (6.9.3.3 [basic.start.dynamic], 8.8 [stmt.dcl])²⁰⁶; or
7. (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.8 [stmt.dcl]).
A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.

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

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