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

Section: 17.13.5 [support.signal], 33.5.10 [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 17.13.5 [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Previous resolution [SUPERSEDED]: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.

1. 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 invoked on an atomic_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 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.

[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 17.13.5 [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.