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Lock-free programming is a technique that allows concurrent updates of shared data structures without using explicit locks. This method ensures that no threads block for arbitrarily long times, and it thereby boosts performance.

AdvantagesLock-free programming has the following advantages:

Can be used in places where locks must be avoided, such as interrupt handlers
Efficiency benefits compared to lock-based algorithms for some workloads, including potential scalability benefits on multiprocessor machines
Avoidance of priority inversion in real-time systems

Limitations:

...

Lock-free programming requires the use of special atomic processor instructions, such as CAS (compare and swap)

...

, LL/SC (load linked/store conditional)

...

, or the C Standard atomic_compare_exchange generic functions.

Applications for lock-free programming includeApplications:

Read-copy-update€ (RCU) in Linux 2.5 kernel
Lock-free programming on AMD multicore systems

The ABA problem occurs during synchronization, when : a memory location is read twice and has the same value for both reads. However, another thread has executed between the two reads and modified the value, did performed other work, then modified the value backback between the two reads, thereby fooling tricking the first thread into thinking that "nothing has changed" even though the second thread did work violating this assumption.

The ABA problem occurs because of the internal reuse of nodes that have been popped off the list or the reclamation of memory occupied by removed nodes.

Noncompliant Code Example (GNU Glib)

the value never changed.

Noncompliant Code Example

This noncompliant code example attempts to zero the maximum element of an array. The example is assumed to run in a multithreaded environment, where all variables are accessed by other threads.This code implements a queue data structure using lock-free programming. It is implemented using glib. The function CAS() internally uses g_atomic_pointer_compare_and_exchange().

Code Block

bgColor	#FFcccc#ffcccc
lang	c

#include <glib<stdatomic.h>
 
/*
 * Sets index to point to index of maximum element in array
 * and value to contain maximum array value.
 */
void find_max_element(atomic_int array[], size_t *index, int *value);

static atomic_int array[];

void func(void) {
  size_t index;
  int value;
  find_max_element(array, &index, &value);
  /* ... */
  if (!atomic_compare_exchange_strong(array[index], &value, 0)) {
    /* Handle error */
  }
}

The compare-and-swap operation sets array[index] to 0 if and only if it is currently set to value. However, this code does not necessarily zero out the maximum value of the array because

index may have changed.
value may have changed (that is, the value of the value variable).
value may no longer be the maximum value in the array.

Compliant Solution (Mutex)

This compliant solution uses a mutex to prevent the data from being modified during the operation. Although this code is thread-safe, it is no longer lock-free.

Code Block

bgColor	#ccccff
lang	c

#include <stdatomic.h>
#include <threads.h>
 
static atomic_int array[];
static mtx_t array_mutex;

void func(void) {
  size_t index;
  int value;
  if (thrd_success != mtx_lock(&array_mutex)) {
    /* Handle error */
  }
  find_max_element(array, &index, &value);
  /* ... */
  if (!atomic_compare_exchange_strong(array[index], &value, 0)) {
    /* Handle error */
  }
  if (thrd_success != mtx_unlock(&array_mutex)#include <glib-object.h>

struct Node {
  void *data;
  Node *next;
};

struct Queue {
  Node *head;
  Node *tail;
};

Queue* queue_new(void) {
  Queue *q = g_slice_new( sizeof(Queue));
  q->head = q->tail = g_slice_new0( sizeof(Node));
  return q;
}

void queue_enqueue(Queue *q, gpointer data) {
  Node *node;
  Node *tail;
  Node *next;

  node = g_slice_new(Node);
  node->data = data;
  node->next = NULL;
  while (TRUE) {
    tail = q->tail;
    next = tail->next;
    if (tail != q->tail) {
      continue;
    }
    if (next != NULL) {
      CAS(&q->tail, tail, next);
      continue;
    }
    if (CAS( &tail->next, null, node)) {
      break;
    }
  }
  CAS(&q->tail, tail, node);
}

gpointer queue_dequeue(Queue *q) {
  Node *node;
  Node *tail;
  Node *next;

  while (TRUE) {
    head = q->head;
    tail = q->tail;
    next = head->next;
    if (head != q->head) {
      continue;
    }
    if (next == NULL) {
    /* Handle return NULL; // Empty
  error */
  }
    if (head == tail) {
      CAS(&q->tail, tail, next);
      continue;
    }
    data = next->data;
    if (CAS(&q->head, head, next)) {
      break;
    }
  }
  g_slice_free(Node, head);
  return data;
}

Assume there are two threads (T1 and T2) operating simultaneously on the queue. The queue looks like this:

head -> A -> B -> C -> tail

The following sequence of operation occurs.

Thread	Queue Before	Operation	Queue After
`T1`	head -> A -> B -> C -> tail	Enters `queue_dequeue()` function head = A, tail = C next = B after executing `data = next->data;` This thread gets preempted	head -> A -> B -> C -> tail
`T2`	head -> A -> B -> C -> tail	Removes node A	head -> B -> C -> tail
`T2`	head -> B -> C -> tail	Removes node B	head -> C -> tail
`T2`	head -> C -> tail	Enqueues node A back into the queue	head -> C -> A -> tail
`T2`	head -> C -> A -> tail	Removes node C	head -> A -> tail
`T2`	head -> A -> tail	Enqueues a new node D After enqueue operation, thread 2 gets preempted	head -> A -> D -> tail
`T1`	head -> A -> D -> tail	Thread 1 starts execution Compares the local head = `q->head` = A (true in this case) Updates `q->head` with node B (but node B is removed)	undefined {}

According to the sequence of events in this tabel, head will now point to memory that was freed. Also, if reclaimed memory is returned to the operating system (for example, using munmap()), access to such memory locations can result in fatal access violation errors.

Compliant Solution (GNU Glib, Hazard Pointers)

The core idea is to associate a number (typically one or two) of single-writer multireader shared pointers, called hazard pointers.

A hazard pointer either has a null value or points to a node that may be accessed later by that thread without further validation that the reference to the node is still valid. Each hazard pointer may be written only by its owner thread but may be read by other threads.

In this solution, communication with the associated algorithms is accomplished only through hazard pointers and a procedure RetireNode() that is called by threads to pass the addresses of retired nodes.

Pseudocode:

Code Block

bgColor	#ccccff
lang	c

// Hazard Pointers Types and Structure
 structure HPRecType {
   HP[K]:*Nodetype; Next:*HPRecType;}
// The header of the HPRec list
 HeadHPRec: *HPRecType;
//Per-thread private variables
 rlist: listType; // initially empty
 rcount: integer; // initially 0


//The Retired Node routine
RetiredNode(node:*NodeType) {
  rlist.push(node);
  rcount++;
  if(rcount >= R)
    Scan(HeadHPRec);
}


// The scan routine
Scan(head:*HPRecType) {
  // Stage1: Scan HP list and insert non-null values in plist
  plist.init();
  hprec<-head;
  while (hprec != null) {
    for (i<-0 to K-1) {
      hptr<-hprec^HP[i];
      if (hptr!= null)
        plist.insert(hptr);
    }
    hprec<-hprec^Next;
  }

  // Stage 2: search plist
  tmplist<-rlist.popAll();
  rcount<-0;
  node<-tmplist.pop();
  while (node != null) {
    if (plist.lookup(node)) {
      rlist.push(node);
      rcount++;
    }
    else {
      PrepareForReuse(node);
    }
    node<-tmplist.pop();
  }
  plist.free();
}

The scan consists of two stages. The first stage involves scanning the hazard pointer list for non-null values. Whenever a non-null value is encountered, it is inserted in a local list, plist, which can be implemented as a hash table. The second stage involves checking each node in rlist against the pointers in plist. If the lookup yields no match, the node is identified to be ready for arbitrary reuse. Otherwise, it is retained in rlist until the next scan by the current thread. Insertion and lookup in plist take constant expected time. The task of the memory reclamation method is to determine when a retired node is eligible for reuse safely while allowing memory reclamation.

In the implementation, the pointer being removed is stored in the hazard pointer, preventing other threads from reusing it and thereby avoiding the ABA problem.

Code:

Noncompliant Code Example (GNU Glib)

This code implements a queue data structure using lock-free programming. It is implemented using glib. The function CAS() internally uses g_atomic_pointer_compare_and_exchange().

Code Block

bgColor	#FFcccc
lang	c

#include <glib.h>
#include <glib-object.h>

typedef struct node_s {
  void *data;
  Node *next;
} Node;

typedef struct queue_s {
  Node *head;
  Node *tail;
} Queue;

Queue* queue_new(void) {
  Queue *q = g_slice_new(sizeof(Queue));
  q->head = q->tail = g_slice_new(sizeof(Node));
  return q;
}

void queue_enqueue(Queue *q, gpointer data) {
  Node *node;
  Node *tail;
  Node *next;

  node = g_slice_new(Node);
  node->data = data;
  node->next = NULL;
  while (TRUE) {
    tail = q->tail;
    next = tail->next;
    if (tail != q->tail) {
      continue;
    }
    if (next != NULL) {
      CAS(&q->tail, tail, next);
      continue;
    }
    if (CAS(&tail->next, NULL, node)) {
      break;
    }
  }
  CAS(&q->tail, tail, node);
}

gpointer queue_dequeue(Queue *q) {
  Node *node;
  Node *head;
  Node *tail;
  Node *next;
  gpointer data;

  while (TRUE) {
    head = q->head;
    tail = q->tail;
    next = head->next;
    if (head != q->head) {
      continue;
    }
    if (next == NULL) {
      return NULL; /* Empty */
    }
    if (head == tail) {
      CAS(&q->tail, tail, next);
      continue;
    }
    data = next->data;
    if (CAS(&q->head, head, next)) {
      break;
    }
  }
  g_slice_free(Node, head);
  return data;
}

Assume there are two threads (T1 and T2) operating simultaneously on the queue. The queue looks like this:

head -> A -> B -> C -> tail

The following sequence of operations occurs:

Thread	Queue Before	Operation	Queue After
`T1`	`head -> A -> B -> C -> tail`	Enters `queue_dequeue()` function `head = A, tail = C` `next = B` after executing `data = next->data;` This thread gets preempted	`head -> A -> B -> C -> tail`
`T2`	`head -> A -> B -> C -> tail`	Removes node A	`head -> B -> C -> tail`
`T2`	`head -> B -> C -> tail`	Removes node B	`head -> C -> tail`
`T2`	`head -> C -> tail`	Enqueues node A back into the queue	`head -> C -> A -> tail`
`T2`	`head -> C -> A -> tail`	Removes node C	`head -> A -> tail`
`T2`	`head -> A -> tail`	Enqueues a new node D After enqueue operation, thread 2 gets preempted	`head -> A -> D -> tail`
`T1`	`head -> A -> D -> tail`	Thread 1 starts execution Compares the local head = `q->head` = A (true in this case) Updates `q->head` with node B (but node B is removed)	undefined {}

According to the sequence of events in this table, head will now point to memory that was freed. Also, if reclaimed memory is returned to the operating system (for example, using munmap()), access to such memory locations can result in fatal access violation errors. The ABA problem occurred because of the internal reuse of nodes that have been popped off the list or the reclamation of memory occupied by removed nodes.

Compliant Solution (GNU Glib, Hazard Pointers)

According to [Michael 2004], the core idea is to associate a number (typically one or two) of single-writer, multi-reader shared pointers, called hazard pointers, with each thread that intends to access lock-free dynamic objects. A hazard pointer either has a null value or points to a node that may be accessed later by that thread without further validation that the reference to the node is still valid. Each hazard pointer may be written only by its owner thread but may be read by other threads.

In this solution, communication with the associated algorithms is accomplished only through hazard pointers and a procedure RetireNode() that is called by threads to pass the addresses of retired nodes.

PSEUDOCODE

Code Block

bgColor	#ccccff
lang	c

/* Hazard pointers types and structure */
structure HPRecType {
  HP[K]:*Nodetype;
  Next:*HPRecType;
}
 
/* The header of the HPRec list */
HeadHPRec: *HPRecType;
/* Per-thread private variables */
rlist: listType; /* Initially empty */
rcount: integer; /* Initially 0 */

/* The retired node routine */
RetiredNode(node:*NodeType) {
  rlist.push(node);
  rcount++;
  if(rcount >= R)
    Scan(HeadHPRec);
}

/* The scan routine */
Scan(head:*HPRecType) {
  /* Stage 1: Scan HP list and insert non-null values in plist */
  plist.init();
  hprec<-head;
  while (hprec != NULL) {
    for (i<-0 to K-1) {
      hptr<-hprec^HP[i];
      if (hptr!= NULL)
        plist.insert(hptr);
    }
    hprec<-hprec^Next;
  }

  /* Stage 2: search plist */
  tmplist<-rlist.popAll();
  rcount<-0;
  node<-tmplist.pop();
  while (node != NULL) {
    if (plist.lookup(node)) {
      rlist.push(node);
      rcount++;
    }
    else {
      PrepareForReuse(node);
    }
    node<-tmplist.pop();
  }
  plist.free();
}

The scan consists of two stages. The first stage involves scanning the hazard pointer list for non-null values. Whenever a non-null value is encountered, it is inserted in a local list, plist, which can be implemented as a hash table. The second stage involves checking each node in rlist against the pointers in plist. If the lookup yields no match, the node is identified to be ready for arbitrary reuse. Otherwise, it is retained in rlist until the next scan by the current thread. Insertion and lookup in plist take constant expected time. The task of the memory reclamation method is to determine when a retired node is safely eligible for reuse while allowing memory reclamation.

In the implementation, the pointer being removed is stored in the hazard pointer, preventing other threads from reusing it and thereby avoiding the ABA problem.

CODE

Code Block

bgColor	#ccccff
lang	c

#include <glib.h>
#include <glib-object.h>
 
void queue_enqueue(Queue *q, gpointer data) {
  Node *node;
  Node *tail;
  Node *next;

  node = g_slice_new(Node);
  node->data = data;
  node->next = NULL;
  while (TRUE) {
    tail = q->tail;
    HAZARD_SET(0, tail);  /* Mark tail as hazardous */
    if (tail != q->tail) {  /* Check tail hasn't changed */
      continue;
    }
    next = tail->next;
    if (tail != q->tail) {
      continue;
    }
    if (next != NULL) {
      CAS(&q->tail, tail, next);
      continue;
    }
    if (CAS(&tail->next, null, node) {
      break;
    }
  }
  CAS(&q->tail, tail, node);
}

gpointer queue_dequeue(Queue *q

Code Block

bgColor	#ccccff
lang	c

void queue_enqueue(Queue *q, gpointer data) {
  Node *node;
  Node *tailhead;
  Node *nexttail;

  node = g_slice_new(Node);
  node->data = data *next;
  node->next = NULLgpointer data;

  while (TRUE) {
    tailhead = q->tail>head;
    LF_HAZARD_SET(0, tailhead);              ///* Mark tailhead as hazardous */
    if (tailhead != q->tail>head) {  /* Check head hasn't changed */
      continue;
    }
  // Check tail hasn't changed= q->tail;
    next = continuehead->next;
    }
LF_HAZARD_SET(1, next);  /* Mark next as = tail->next;hazardous */
    if (tailhead != q->tail>head) {
      continue;
    }
    if (next !== NULL) {
      CAS(&q->tail, tail, next);
      continue;return NULL; /* Empty */
    }
    if (CAS(&tail->next, null, nodehead == tail) {
      break;
    }
  }
  CAS(&q->tail, tail, node);
}


gpointer queue_dequeue(Queue *q) {
  Node *nodenext);
  Node *tail;
  Node *nextcontinue;

  while (TRUE) {}
    headdata = qnext->head>data;
     LF_HAZARD_SET(0if (CAS(&q->head, head, next));  {
  // Mark head as hazardousbreak;
    if}
 (head != q->head) {    }
  LF_HAZARD_UNSET(head);  //*
 Check head hasn't changed
      continue;
    }
    tail = q->tail;
    next = head->next;
 *   LF_HAZARD_SET(1, next); Retire head, and perform
   // Mark next as hazardous
    if (head != q->head) {
      continue;
    }
  * reclamation if (next == NULL) {
needed.
           return NULL; // Empty
    }
    if (head == tail) { */
      CAS(&q->tail, tail, next);
      continue;
    }
    data = next->data;
    if (CAS(&q->head, head, next)) {
      break;
    }
  }
  LF_HAZARD_UNSET(head);        // Retire head, and perform
                                // reclamation if needed.
  return data;
}

Compliant Solution (GNU Glib, Mutex)

In this compliant solution, mtx_lock() is used to lock the queue.

When thread 1 locks on the queue to perform any operation, thread 2 cannot perform any operation on the queue, which prevents the ABA problem.

return data;
}

Compliant Solution (GNU Glib, Mutex)

In this compliant solution, mtx_lock() is used to lock the queue. When thread 1 locks on the queue to perform any operation, thread 2 cannot perform any operation on the queue, which prevents the ABA problem.

Code Block

bgColor	#ccccff
lang	c

#include <threads.h>
#include <glib-object.h>

typedef struct node_s {
  void *data;
  Node *next;
} Node;

typedef struct queue_s {
  Node *head;
  Node *tail;
  mtx_t mutex;
} Queue;

Queue* queue_new(void) {
  Queue *q = g_slice_new(sizeof(Queue));
  q->head = q->tail = g_slice_new(sizeof(Node));
  return q;
}

int queue_enqueue(Queue *q, gpointer data) {
  Node *node;
  Node *tail;
  Node *next;

  /*
   * Lock the queue before accessing the contents and
   * check the return code for success.
   */
  if (thrd_success != mtx_lock(&(q->mutex))) {
    return -1;  /* Indicate failure */
  } else {
    node

Code Block

bgColor	#ccccff
lang	c

#include <glib-object.h>

struct Node {
  void *data;
  Node *next;
};

struct Queue {
  Node *head;
  Node *tail;
  mtx_t mutex;
};

Queue* queue_new(void) {
  Queue *q = g_slice_new( sizeof(Queue)Node);
  q->head = qnode->tail>data = g_slice_new0( sizeof(Node))data;
    node->next return= qNULL;
}

void queue_enqueue(Queue *q, gpointer data    if(q->head == NULL) {
  Node *node;
  Node *tail;
  Node *next;
 q->head = node;
  // Lock the queue beforeq->tail accessing the contents= node;
  mtx_lock(&(q->mutex));

  node} = g_slice_new(Node);else {
  node->data = data;
  nodeq->tail->next = NULLnode;

      if(q->head>tail == NULL) {
    q->head = node;
    q->tail = node;
  }
  else {
    q->tail->next = node;
    q->tail = node;
 node;
    }
    /* Unlock the mutex and check the return code */
    if (thrd_success != mtx_unlock(&(queue->mutex))) {
      return -1;  /* Indicate failure */
    }
  //}
 Unlock the mutex
  mtx_unlock(&(queue->mutex))return 0;
}

gpointer queue_dequeue(Queue *q) {
  Node *node;
  Node *head;
  Node *tail;
  Node *next;
  gpointer data;

  if (thrd_success != mtx_lock(&(q->mutex)) {
    return NULL;  /* Indicate failure */
  } else {
    head = q->head;
    tail = q->tail;
    next = head->next;
    data = next->data;
    q->head = next;

    g_slice_free(Node, head);
    if (thrd_success != mtx_unlock(&(queue->mutex))) {
      return NULL;  /* Indicate failure */
    }
  }
  return data;
}

Risk Assessment

The likelihood of having a race condition is low. Once the race condition occurs, the reading memory that has already been freed can lead to abnormal program termination or unintended information disclosure.

Recommendation

Guideline

Severity

Severity

Likelihood

Likelihood

Detectable

Remediation Cost

Repairable	Priority	Level

CON39

CON09-C

Medium

Unlikely

unlikely

No

High

No

P2

L3

Sources

Michael, M.M. "Hazard pointers: Safe memory reclamation for lock-free objects," IEEE Transactions on Parallel and Distributed Systems 15, 8 (2004)

...

Automated Detection

Tool	Version	Checker	Description

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

[Apiki 2006]

"Lock-Free Programming on AMD Multi-Core System"

...

[Asgher 2000]

"Practical Lock-Free Buffers

...

"
[Michael 2004]	"Hazard Pointers"

...

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Noncompliant Code Example (GNU Glib)

Noncompliant Code Example

Compliant Solution (Mutex)

Compliant Solution (GNU Glib, Hazard Pointers)

Noncompliant Code Example (GNU Glib)

Compliant Solution (GNU Glib, Hazard Pointers)

PSEUDOCODE

CODE

Compliant Solution (GNU Glib, Mutex)

Compliant Solution (GNU Glib, Mutex)

Risk Assessment

Sources

Automated Detection

Related Vulnerabilities

Bibliography