[intro.multithread] - C++20 → C++23

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@@ -1,22 +1,25 @@
 ### Multi-threaded executions and data races <a id="intro.multithread">[[intro.multithread]]</a>
 A *thread of execution* (also known as a *thread*) is a single flow of
 control within a program, including the initial invocation of a specific
 top-level function, and recursively including every function invocation
 subsequently executed by the thread.
 [*Note 1*: When one thread creates another, the initial call to the
 top-level function of the new thread is executed by the new thread, not
 by the creating thread. — *end note*]
 Every thread in a program can potentially access every object and
-function in a program.[^28] Under a hosted implementation, a C++ program
-can have more than one thread running concurrently. The execution of
-each thread proceeds as defined by the remainder of this document. The
-execution of the entire program consists of an execution of all of its
-threads.
 [*Note 2*: Usually the execution can be viewed as an interleaving of
 all its threads. However, some kinds of atomic operations, for example,
 allow executions inconsistent with a simple interleaving, as described
 below. — *end note*]
@@ -33,12 +36,12 @@ contains the signal handler invocation.
 The value of an object visible to a thread T at a particular point is
 the initial value of the object, a value assigned to the object by T, or
 a value assigned to the object by another thread, according to the rules
 below.
-[*Note 1*: In some cases, there may instead be undefined behavior. Much
-of this subclause is motivated by the desire to support atomic
 operations with explicit and detailed visibility constraints. However,
 it also implicitly supports a simpler view for more restricted
 programs. — *end note*]
 Two expression evaluations *conflict* if one of them modifies a memory
@@ -71,11 +74,11 @@ All modifications to a particular atomic object M occur in some
 particular total order, called the *modification order* of M.
 [*Note 3*: There is a separate order for each atomic object. There is
 no requirement that these can be combined into a single total order for
 all objects. In general this will be impossible since different threads
-may observe modifications to different objects in inconsistent
 orders. — *end note*]
 A *release sequence* headed by a release operation A on an atomic object
 M is a maximal contiguous sub-sequence of side effects in the
 modification order of M, where the first operation is A, and every
@@ -284,12 +287,12 @@ computation of an object being taken from the last side effect on that
 object in that interleaving. This is normally referred to as “sequential
 consistency”. However, this applies only to data-race-free programs, and
 data-race-free programs cannot observe most program transformations that
 do not change single-threaded program semantics. In fact, most
 single-threaded program transformations continue to be allowed, since
-any program that behaves differently as a result must perform an
-undefined operation. — *end note*]
 Two accesses to the same object of type `volatile std::sig_atomic_t` do
 not result in a data race if both occur in the same thread, even if one
 or more occurs in a signal handler. For each signal handler invocation,
 evaluations performed by the thread invoking a signal handler can be
@@ -305,17 +308,17 @@ potentially shared memory location that would not be modified by the
 abstract machine are generally precluded by this document, since such an
 assignment might overwrite another assignment by a different thread in
 cases in which an abstract machine execution would not have encountered
 a data race. This includes implementations of data member assignment
 that overwrite adjacent members in separate memory locations. Reordering
-of atomic loads in cases in which the atomics in question may alias is
-also generally precluded, since this may violate the coherence
 rules. — *end note*]
 [*Note 23*: Transformations that introduce a speculative read of a
-potentially shared memory location may not preserve the semantics of the
-C++ program as defined in this document, since they potentially
 introduce a data race. However, they are typically valid in the context
 of an optimizing compiler that targets a specific machine with
 well-defined semantics for data races. They would be invalid for a
 hypothetical machine that is not tolerant of races or provides hardware
 race detection. — *end note*]
@@ -338,25 +341,25 @@ Executions of atomic functions that are either defined to be lock-free
 [[atomics.flag]] or indicated as lock-free [[atomics.lockfree]] are
 *lock-free executions*.
 - If there is only one thread that is not blocked [[defns.block]] in a
   standard library function, a lock-free execution in that thread shall
-  complete. \[*Note 2*: Concurrently executing threads may prevent
   progress of a lock-free execution. For example, this situation can
   occur with load-locked store-conditional implementations. This
   property is sometimes termed obstruction-free. — *end note*]
 - When one or more lock-free executions run concurrently, at least one
   should complete. \[*Note 3*: It is difficult for some implementations
   to provide absolute guarantees to this effect, since repeated and
-  particularly inopportune interference from other threads may prevent
   forward progress, e.g., by repeatedly stealing a cache line for
   unrelated purposes between load-locked and store-conditional
-  instructions. Implementations should ensure that such effects cannot
- indefinitely delay progress under expected operating conditions, and
- that such anomalies can therefore safely be ignored by programmers.
-  Outside this document, this property is sometimes termed
-  lock-free. — *end note*]
 During the execution of a thread of execution, each of the following is
 termed an *execution step*:
 - termination of the thread of execution,
@@ -368,11 +371,11 @@ An invocation of a standard library function that blocks [[defns.block]]
 is considered to continuously execute execution steps while waiting for
 the condition that it blocks on to be satisfied.
 [*Example 1*: A library I/O function that blocks until the I/O
 operation is complete can be considered to continuously check whether
-the operation is complete. Each such check might consist of one or more
 execution steps, for example using observable behavior of the abstract
 machine. — *end example*]
 [*Note 4*: Because of this and the preceding requirement regarding what
 threads of execution have to perform eventually, it follows that no
@@ -387,30 +390,28 @@ concurrent threads that are not blocked in a standard library function
 For a thread of execution providing *concurrent forward progress
 guarantees*, the implementation ensures that the thread will eventually
 make progress for as long as it has not terminated.
 [*Note 5*: This is required regardless of whether or not other threads
-of executions (if any) have been or are making progress. To eventually
 fulfill this requirement means that this will happen in an unspecified
 but finite amount of time. — *end note*]
 It is *implementation-defined* whether the implementation-created thread
 of execution that executes `main` [[basic.start.main]] and the threads
 of execution created by `std::thread` [[thread.thread.class]] or
 `std::jthread` [[thread.jthread.class]] provide concurrent forward
-progress guarantees.
-[*Note 6*: General-purpose implementations should provide these
-guarantees. — *end note*]
 For a thread of execution providing *parallel forward progress
 guarantees*, the implementation is not required to ensure that the
 thread will eventually make progress if it has not yet executed any
 execution step; once this thread has executed a step, it provides
 concurrent forward progress guarantees.
-[*Note 7*: This does not specify a requirement for when to start this
 thread of execution, which will typically be specified by the entity
 that creates this thread of execution. For example, a thread of
 execution that provides concurrent forward progress guarantees and
 executes tasks from a set of tasks in an arbitrary order, one after the
 other, satisfies the requirements of parallel forward progress for these
@@ -418,57 +419,57 @@ tasks. — *end note*]
 For a thread of execution providing *weakly parallel forward progress
 guarantees*, the implementation does not ensure that the thread will
 eventually make progress.
-[*Note 8*: Threads of execution providing weakly parallel forward
 progress guarantees cannot be expected to make progress regardless of
 whether other threads make progress or not; however, blocking with
 forward progress guarantee delegation, as defined below, can be used to
 ensure that such threads of execution make progress
 eventually. — *end note*]
 Concurrent forward progress guarantees are stronger than parallel
 forward progress guarantees, which in turn are stronger than weakly
 parallel forward progress guarantees.
-[*Note 9*: For example, some kinds of synchronization between threads
-of execution may only make progress if the respective threads of
 execution provide parallel forward progress guarantees, but will fail to
 make progress under weakly parallel guarantees. — *end note*]
 When a thread of execution P is specified to *block with forward
 progress guarantee delegation* on the completion of a set S of threads
 of execution, then throughout the whole time of P being blocked on S,
 the implementation shall ensure that the forward progress guarantees
 provided by at least one thread of execution in S is at least as strong
 as P’s forward progress guarantees.
-[*Note 10*: It is unspecified which thread or threads of execution in S
 are chosen and for which number of execution steps. The strengthening is
 not permanent and not necessarily in place for the rest of the lifetime
 of the affected thread of execution. As long as P is blocked, the
 implementation has to eventually select and potentially strengthen a
 thread of execution in S. — *end note*]
 Once a thread of execution in S terminates, it is removed from S. Once S
 is empty, P is unblocked.
-[*Note 11*: A thread of execution B thus can temporarily provide an
 effectively stronger forward progress guarantee for a certain amount of
 time, due to a second thread of execution A being blocked on it with
 forward progress guarantee delegation. In turn, if B then blocks with
-forward progress guarantee delegation on C, this may also temporarily
 provide a stronger forward progress guarantee to C. — *end note*]
-[*Note 12*: If all threads of execution in S finish executing (e.g.,
 they terminate and do not use blocking synchronization incorrectly),
 then P’s execution of the operation that blocks with forward progress
 guarantee delegation will not result in P’s progress guarantee being
 effectively weakened. — *end note*]
-[*Note 13*: This does not remove any constraints regarding blocking
 synchronization for threads of execution providing parallel or weakly
 parallel forward progress guarantees because the implementation is not
 required to strengthen a particular thread of execution whose too-weak
 progress guarantee is preventing overall progress. — *end note*]

 ### Multi-threaded executions and data races <a id="intro.multithread">[[intro.multithread]]</a>
+#### General <a id="intro.multithread.general">[[intro.multithread.general]]</a>
 A *thread of execution* (also known as a *thread*) is a single flow of
 control within a program, including the initial invocation of a specific
 top-level function, and recursively including every function invocation
 subsequently executed by the thread.
 [*Note 1*: When one thread creates another, the initial call to the
 top-level function of the new thread is executed by the new thread, not
 by the creating thread. — *end note*]
 Every thread in a program can potentially access every object and
+function in a program.[^24]
+Under a hosted implementation, a C++ program can have more than one
+thread running concurrently. The execution of each thread proceeds as
+defined by the remainder of this document. The execution of the entire
+program consists of an execution of all of its threads.
 [*Note 2*: Usually the execution can be viewed as an interleaving of
 all its threads. However, some kinds of atomic operations, for example,
 allow executions inconsistent with a simple interleaving, as described
 below. — *end note*]
 The value of an object visible to a thread T at a particular point is
 the initial value of the object, a value assigned to the object by T, or
 a value assigned to the object by another thread, according to the rules
 below.
+[*Note 1*: In some cases, there might instead be undefined behavior.
+Much of this subclause is motivated by the desire to support atomic
 operations with explicit and detailed visibility constraints. However,
 it also implicitly supports a simpler view for more restricted
 programs. — *end note*]
 Two expression evaluations *conflict* if one of them modifies a memory
 particular total order, called the *modification order* of M.
 [*Note 3*: There is a separate order for each atomic object. There is
 no requirement that these can be combined into a single total order for
 all objects. In general this will be impossible since different threads
+can observe modifications to different objects in inconsistent
 orders. — *end note*]
 A *release sequence* headed by a release operation A on an atomic object
 M is a maximal contiguous sub-sequence of side effects in the
 modification order of M, where the first operation is A, and every
 object in that interleaving. This is normally referred to as “sequential
 consistency”. However, this applies only to data-race-free programs, and
 data-race-free programs cannot observe most program transformations that
 do not change single-threaded program semantics. In fact, most
 single-threaded program transformations continue to be allowed, since
+any program that behaves differently as a result has undefined
+behavior. — *end note*]
 Two accesses to the same object of type `volatile std::sig_atomic_t` do
 not result in a data race if both occur in the same thread, even if one
 or more occurs in a signal handler. For each signal handler invocation,
 evaluations performed by the thread invoking a signal handler can be
 abstract machine are generally precluded by this document, since such an
 assignment might overwrite another assignment by a different thread in
 cases in which an abstract machine execution would not have encountered
 a data race. This includes implementations of data member assignment
 that overwrite adjacent members in separate memory locations. Reordering
+of atomic loads in cases in which the atomics in question might alias is
+also generally precluded, since this could violate the coherence
 rules. — *end note*]
 [*Note 23*: Transformations that introduce a speculative read of a
+potentially shared memory location might not preserve the semantics of
+the C++ program as defined in this document, since they potentially
 introduce a data race. However, they are typically valid in the context
 of an optimizing compiler that targets a specific machine with
 well-defined semantics for data races. They would be invalid for a
 hypothetical machine that is not tolerant of races or provides hardware
 race detection. — *end note*]
 [[atomics.flag]] or indicated as lock-free [[atomics.lockfree]] are
 *lock-free executions*.
 - If there is only one thread that is not blocked [[defns.block]] in a
   standard library function, a lock-free execution in that thread shall
+  complete. \[*Note 2*: Concurrently executing threads might prevent
   progress of a lock-free execution. For example, this situation can
   occur with load-locked store-conditional implementations. This
   property is sometimes termed obstruction-free. — *end note*]
 - When one or more lock-free executions run concurrently, at least one
   should complete. \[*Note 3*: It is difficult for some implementations
   to provide absolute guarantees to this effect, since repeated and
+  particularly inopportune interference from other threads could prevent
   forward progress, e.g., by repeatedly stealing a cache line for
   unrelated purposes between load-locked and store-conditional
+  instructions. For implementations that follow this recommendation and
+ ensure that such effects cannot indefinitely delay progress under
+ expected operating conditions, such anomalies can therefore safely be
+ ignored by programmers. Outside this document, this property is
+ sometimes termed lock-free. — *end note*]
 During the execution of a thread of execution, each of the following is
 termed an *execution step*:
 - termination of the thread of execution,
 is considered to continuously execute execution steps while waiting for
 the condition that it blocks on to be satisfied.
 [*Example 1*: A library I/O function that blocks until the I/O
 operation is complete can be considered to continuously check whether
+the operation is complete. Each such check consists of one or more
 execution steps, for example using observable behavior of the abstract
 machine. — *end example*]
 [*Note 4*: Because of this and the preceding requirement regarding what
 threads of execution have to perform eventually, it follows that no
 For a thread of execution providing *concurrent forward progress
 guarantees*, the implementation ensures that the thread will eventually
 make progress for as long as it has not terminated.
 [*Note 5*: This is required regardless of whether or not other threads
+of execution (if any) have been or are making progress. To eventually
 fulfill this requirement means that this will happen in an unspecified
 but finite amount of time. — *end note*]
 It is *implementation-defined* whether the implementation-created thread
 of execution that executes `main` [[basic.start.main]] and the threads
 of execution created by `std::thread` [[thread.thread.class]] or
 `std::jthread` [[thread.jthread.class]] provide concurrent forward
+progress guarantees. General-purpose implementations should provide
+these guarantees.
 For a thread of execution providing *parallel forward progress
 guarantees*, the implementation is not required to ensure that the
 thread will eventually make progress if it has not yet executed any
 execution step; once this thread has executed a step, it provides
 concurrent forward progress guarantees.
+[*Note 6*: This does not specify a requirement for when to start this
 thread of execution, which will typically be specified by the entity
 that creates this thread of execution. For example, a thread of
 execution that provides concurrent forward progress guarantees and
 executes tasks from a set of tasks in an arbitrary order, one after the
 other, satisfies the requirements of parallel forward progress for these
 For a thread of execution providing *weakly parallel forward progress
 guarantees*, the implementation does not ensure that the thread will
 eventually make progress.
+[*Note 7*: Threads of execution providing weakly parallel forward
 progress guarantees cannot be expected to make progress regardless of
 whether other threads make progress or not; however, blocking with
 forward progress guarantee delegation, as defined below, can be used to
 ensure that such threads of execution make progress
 eventually. — *end note*]
 Concurrent forward progress guarantees are stronger than parallel
 forward progress guarantees, which in turn are stronger than weakly
 parallel forward progress guarantees.
+[*Note 8*: For example, some kinds of synchronization between threads
+of execution might only make progress if the respective threads of
 execution provide parallel forward progress guarantees, but will fail to
 make progress under weakly parallel guarantees. — *end note*]
 When a thread of execution P is specified to *block with forward
 progress guarantee delegation* on the completion of a set S of threads
 of execution, then throughout the whole time of P being blocked on S,
 the implementation shall ensure that the forward progress guarantees
 provided by at least one thread of execution in S is at least as strong
 as P’s forward progress guarantees.
+[*Note 9*: It is unspecified which thread or threads of execution in S
 are chosen and for which number of execution steps. The strengthening is
 not permanent and not necessarily in place for the rest of the lifetime
 of the affected thread of execution. As long as P is blocked, the
 implementation has to eventually select and potentially strengthen a
 thread of execution in S. — *end note*]
 Once a thread of execution in S terminates, it is removed from S. Once S
 is empty, P is unblocked.
+[*Note 10*: A thread of execution B thus can temporarily provide an
 effectively stronger forward progress guarantee for a certain amount of
 time, due to a second thread of execution A being blocked on it with
 forward progress guarantee delegation. In turn, if B then blocks with
+forward progress guarantee delegation on C, this can also temporarily
 provide a stronger forward progress guarantee to C. — *end note*]
+[*Note 11*: If all threads of execution in S finish executing (e.g.,
 they terminate and do not use blocking synchronization incorrectly),
 then P’s execution of the operation that blocks with forward progress
 guarantee delegation will not result in P’s progress guarantee being
 effectively weakened. — *end note*]
+[*Note 12*: This does not remove any constraints regarding blocking
 synchronization for threads of execution providing parallel or weakly
 parallel forward progress guarantees because the implementation is not
 required to strengthen a particular thread of execution whose too-weak
 progress guarantee is preventing overall progress. — *end note*]

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