CON42-J. Ensure that compound operations on shared variables are atomic

Compound operations are operations that consist of more than one discrete operation. Expressions that include postfix or prefix increment (++), postfix or prefix decrement (--), or compound assignment operators always result in compound operations. Compound assignment expressions use operators such as *=, /=, %=, +=, -=, <<=, >>=, ^= and |=. Compound operations on shared variables must be performed atomically to prevent data races.

Noncompliant Code Example (Logical Negation)

This noncompliant code example declares a shared _Bool flag variable and provides a toggle_flag() method that negates the current value of flag.

static _Bool flag = 0;
 
void toggle_flag() {
  flag = !flag;
}
 
_Bool get_flag() {
  return flag;
}

Execution of this code may result in a data race because the value of flag is read, negated, and written back.

Consider, for example, two threads that call toggle_flag(). The expected effect of toggling flag twice is that it is restored to its original value. However, the following scenario leaves flag in the incorrect state:

As a result, the effect of the call by t₂ is not reflected in flag; the program behaves as if toggle_flag() was called only once, not twice.

Noncompliant Code Example (Bitwise Negation)

The toggle_flag() method may also use the compound assignment operator ^= to negate the current value of flag.

static _Bool flag = 0;
 
void toggle_flag() {
  flag ^= 1;
}
 
_Bool get_flag() {
  return flag;
}

Compliant Solution (Mutex)

This compliant solution restricts access to flag under a mutex lock.

static _Bool flag = 0;
mtx_t flag_mutex;
int result;

/* initialize flag_mutex */
if ((result = mtx_init(&flag_mutex, mtx_plain)) == thrd_error) {
  /* handle error */
}
 
void toggle_flag() {
  int result;
  if ((result = mtx_lock(&flag_mutex)) != thrd_success) {
    /* handle error */
  }
  flag ^= 1;
  if ((result = mtx_unlock(&flag_mutex)) != thrd_success) {
    /* handle error */
  }
}
 
_Bool get_flag() {
  int result;
  _Bool temp_flag;
  if ((result = mtx_lock(&flag_mutex)) != thrd_success) {
    /* handle error */
  }
  temp_flag = flag;
  if ((result = mtx_unlock(&flag_mutex)) != thrd_success) {
    /* handle error */
  }
  return temp_flag;
}

The second execution order involves the same operations, but t₂ starts and finishes before t₁. 

Noncompliant Code Example (atomic boolean)

This noncompliant code example declares flag to be of type _Atomic _Bool.

static _Atomic _Bool flag = 0;
  
void toggle_flag() {
  _Bool temp_flag = atomic_load(&flag);
  temp_flag = !temp_flag;
  atomic_store( &flag, temp_flag);
}
  
_Bool get_flag() {
  return atomic_load(&flag);
}

Compliant Solution (atomic boolean)

This compliant solution declares flag to be of type _Atomic _Bool.

static _Atomic _Bool flag = 0;
  
void toggle_flag() {
  _Bool temp_flag = atomic_load(&flag);
  temp_flag = !temp_flag;
  _bool dummy = 1;
  atomic_compare_exchange_strong( &flag, &dummy, temp_flag);
}
  
_Bool get_flag() {
  return atomic_load(&flag);
}

Noncompliant Code Example (Addition of Primitives)

In this noncompliant code example, multiple threads can invoke the setValues() method to set the a and b fields. Because this class fails to test for integer overflow, users of the Adder class must ensure that the arguments to the setValues() method can be added without overflow. (See rule NUM00-J. Detect or prevent integer overflow for more information.)

The getSum() method contains a race condition. For example, when a and b currently have the values 0 and Integer.MAX_VALUE, respectively, and one thread calls getSum() while another calls setValues(Integer.MAX_VALUE, 0), the getSum() method might return either 0 or Integer.MAX_VALUE, or it might overflow. Overflow will occur when the first thread reads a and b after the second thread has set the value of a to Integer.MAX_VALUE, but before it has set the value of b to 0.

Note that declaring the variables as volatile fails to resolve the issue because these compound operations involve reads and writes of multiple variables.

Noncompliant Code Example (Addition of Atomic Integers)

In this noncompliant code example, a and b are replaced with atomic integers.

The simple replacement of the two int fields with atomic integers fails to eliminate the race condition because the compound operation a.get() + b.get() is still non-atomic.

Compliant Solution (Addition)

This compliant solution synchronizes the setValues() and getSum() methods to ensure atomicity.

The operations within the synchronized methods are now atomic with respect to other synchronized methods that lock on that object's monitor (that is, it's intrinsic lock). It is now possible, for example, to add overflow checking to the synchronized getSum() method without introducing the possibility of a race condition.

Risk Assessment

When operations on shared variables are not atomic, unexpected results can be produced. For example, information can be disclosed inadvertently because one user can receive information about other users.

Automated Detection

Some available static analysis tools can detect the instances of nonatomic update of a concurrently shared value. The result of the update is determined by the interleaving of thread execution. These tools can detect the instances where thread-shared data is accessed without holding an appropriate lock, possibly causing a race condition.

Related Guidelines

Bibliography