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Functions that can fail spuriously should be wrapped in a loop.  The atomic_compare_exchange_weak() and atomic_compare_exchange_weak_explicit() functions both attempt to set an atomic variable to a new value but only if it currently possesses a known old value. Unlike the related functions atomic_compare_exchange_strong() and atomic_compare_exchange_strong_explicit(), these functions are permitted to fail spuriously, which makes them faster on some platforms, for example, architectures that implement compare-and-exchange using load-linked/store-conditional instructions, such as Alpha, ARM, MIPS, and PowerPC. The C Standard describes this behavior in subclause 7.17.7.4, paragraph 4:

A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by expected and object are equal, it may return zero and store back to expected the same memory contents that were originally there.

Noncompliant Code Example

In this noncompliant code example, reorganize_data_structure() is intended to be used as an argument to thrd_create().  After reorganizing, the function attempts to replace the head pointer so that it points to the new version.  If no other thread has changed the head pointer since it was originally loaded, reorganize_data_structure() is intended to exit the thread with a result of true, indicating success.  Otherwise, the new reorganization attempt is discarded and the thread is exited with a result of false.  However, atomic_compare_exchange_weak() may fail even when the head pointer has not changed.  Therefore, reorganize_data_structure() may perform the work and then discard it unnecessarily.

#include <stdatomic.h>
#include <stdbool.h>

struct data_structure {
  struct data_structure *next;
  /* ... */
};

extern void cleanup_data_structure(struct data_structure *head);

int reorganize_data_structure(void *thread_arg) {
  struct data_structure *_Atomic *ptr_to_head = thread_arg;
  struct data_structure *old_head = atomic_load(ptr_to_head);
  struct data_structure *new_head;
  bool success;

  /* ... reorganize the data structure ... */

  success = atomic_compare_exchange_weak(ptr_to_head,
                                         &old_head,
                                         new_head);
  if (!success) {
    cleanup_data_structure(new_head);
  }
  return success; /* exit the thread */
}

Compliant Solution (atomic_compare_exchange_weak())

To recover from spurious failures, a loop must be used.  However, atomic_compare_exchange_weak() might fail because the head pointer changed or the failure may be spurious.  In either case, the thread must perform the work repeatedly until the compare-and-exchange succeeds. 

#include <stdatomic.h>
#include <stdbool.h>
#include <stddef.h>

struct data_structure {
  struct data_structure *next;
  /* ... */
};

extern void cleanup_data_structure(struct data_structure *head);

int reorganize_data_structure(void *thread_arg) {
  struct data_structure *_Atomic *ptr_to_head = thread_arg;
  struct data_structure *old_head = atomic_load(ptr_to_head);
  struct data_structure *new_head = NULL;
  struct data_structure *saved_old_head;
  bool success;

  do {
    if (new_head != NULL) {
      cleanup_data_structure(new_head);
    }
    saved_old_head = old_head;

  /* ... reorganize the data structure ... */

  } while (!(success = atomic_compare_exchange_weak(ptr_to_head,
                                                    &old_head,
                                                    new_head))
 &&
          && old_head == saved_old_head);
  return success; /* exit the thread */
}

Compliant Solution (atomic_compare_exchange_strong())

When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

#include <stdatomic.h>
#include <stdbool.h>

struct data_structure {
  struct data_structure *next;
  /* ... */
};

extern void cleanup_data_structure(struct data_structure *head);

int reorganize_data_structure(void *thread_arg) {
  struct data_structure *_Atomic *ptr_to_head = thread_arg;
  struct data_structure *old_head = atomic_load(ptr_to_head);
  struct data_structure *new_head;
  bool success;

  /* ... reorganize the data structure ... */

  success = atomic_compare_exchange_strong(ptr_to_head,
                                           &old_head,
                                           new_head);
  if (!success) {
    cleanup_data_structure(new_head);
  }
  return success; /* exit the thread */
}

Exceptions

CON44-EX1: Functions such as the atomic_compare_exchange_weak() and atomic_compare_exchange_weak_explicit() that can fail spuriously are typically wrapped in a loop until the function succeeds.  However, this loop could be part of a larger control flow.  In the noncompliant example given for this rule, the thread could be retried if it returns false, for example.

Failing to wrap the atomic_compare_exchange_weak() and atomic_compare_exchange_weak_explicit() functions in a loop can result in incorrect values and control flow.

Related Guidelines

Bibliography