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Lock-Free Programming free programming is a technique that allows concurrent update updates of shared data structures without using explicit locks.   This method ensures system wide progress.

Advantages:

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that no threads block for arbitrarily long times, and it thereby boosts performance.

Lock-free programming has the following advantages:

• Can be used in places

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• where locks

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• must be avoided

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• , such as interrupt handlers
• Efficiency benefits compared to lock-based algorithms

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• for some workloads, including potential scalability benefits on multiprocessor machines

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• Avoidance of priority inversion in real-time systems

Limitations:

-         use Lock-free programming requires the use of special atomic processor instructions, such as CAS (compare and swap) or , LL/SC (load linked/store conditional)

Applications:

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, or the C Standard atomic_compare_exchange generic functions.

Applications for lock-free programming include

• Read-copy-

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• update€ (RCU) in Linux 2.5 kernel

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• Lock-

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• free programming on AMD

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Problem Associated with Lock-Free Approach(ABA):

• multicore systems

The ABA problem occurs during synchronization: a memory ABA problem occurs during synchronization, when a location is read twice , and has the same value for both reads. However, another thread can execute between the two reads and change the has modified the value, do performed other work, then change modified the value back, thus fooling back between the two reads, thereby tricking the first thread in to thinking "nothing has changed" even though the second thread did work that violates that assumption.

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

Noncompliant Code Example (Queue)

into thinking that 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 threadsA queue data structure implementation using lock free method.

#include <glib.h>#include <glib.h>
#include <glib-object.h>
#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, *tail, *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, *tail, *next;
  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;
}

The above implementation works with glib. The function CAS uses g_atomic_pointer_compare_and_exchange() internallyLet's consider the following example: 

Let us 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.

According to the above sequence of events now head will be pointing to a memory which was removed.

Also  if reclaimed memory(B) is returned  to  the operating  system  (e.g., using  munmap), access to such memory locations can result in fatal access violation  errors.

Compliant Solution (using Hazard pointers)

The core idea is to associate a number (typically one or two)  of single-writer  multi-reader  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 can be written only by its owner thread, but can be read by other threads.

This methodology communicates with the associated algorithms only through hazard pointers and a procedure RetireNode that is called by threads to pass the addresses of retired nodes.

Pseudo Code:

// Hazard Pointers Types and Strucutre
 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  HP  list  for non-null values. Whenever a nonnull value is encountered, it is inserted in a local list plist, which can be implemented as a hash table. The second stage of Scan 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.

Code:

<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.

#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)) {
    /* Handle error */
  }
}

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().

#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:

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

/* 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

#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) {
  Node *node;
  Node *head;
  Node *tail;
  Node *next;
  gpointer data;

  while (TRUE) {
    head = q->head;
    LF_HAZARD_SET(0, head);  /* Mark head as hazardous */
    if (head != q->head) {  /* Check head hasn't changed */
      continue;
    }
    tail = q->tail;
    next = head->next;
    LF_HAZARD_SET(1, next);  /* Mark next as hazardous */
    if (head != q->head) {

 void queue_enqueue(Queue *q, gpointer data) {void queue_enqueue(Queue *q, gpointer data) {
  Node *node, *tail, *next;
  node = g_slice_new(Node);
  node->data = data;
  node->next = NULL;
  while (TRUE) {
    tail = q->tail;
    HAZARD_SET(0, tail);              // Mark tail has hazardous
    if (tail != q->tail)             // Check tail hasn't changed
      continue;
    next = tail->next;}
    if (tailnext !== q->tailNULL) {
      return continue;NULL; /* Empty */
    }
    if (nexthead !== NULLtail) {
      CAS(&q->tail, tail, next);
      continue;
    }
    if (CAS(&tail->next, null, node)
      break;
  }
  CAS(&q->tail, tail, node);
}


gpointer queue_dequeue(Queue *q) {
  Node *tail, *next, *head;
  while (TRUEdata = next->data;
    if (CAS(&q->head, head, next)) {
    head = q->head;break;
    }
  }
  LF_HAZARD_SETUNSET(0, head);  /*
  // Mark head as hazardous
    if (head != q->head)         // Check head hasn't changed
  * Retire head, and continue;perform
    tail = q->tail;
    next = head->next;
    LF_HAZARD_SET(1, next);    // Mark next has hazardous
  * reclamation if (head != q->head)needed.
      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;
  }
  LF_HAZARD_UNSET(head);        // Retire head, and perform
                                       // reclamation if needed.
  return data;
}
5 Node *node, *tail, *next;
7 node = g_slice_new(Node);

In the above code, the pointer on being removed is stored in the hazard pointer and thus prevents other threads from reusing it and avoiding the ABA problem.

Compliant Solution (using Mutex)

Code:

 */
  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.

#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 = g_slice_new(Node);
    node->data = data;
    node->next = NULL;

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

gpointer 

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

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

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

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, *tail, *next;
  // Lock the queue before accesing the contents
  pthread_mutex_lock(&(q->mutex));

  node = g_slice_new(Node);
  node->data = data;
  node->next = NULL;

  if(q->head == NULL){
    q->head = node;
    q->tail = node;
  }
  else {
    q->tail->next = node;
    q->tail = node;
  }
  //Unlock the mutex
  pthread_mutex_unlock(&(queue->mutex));
}

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

  pthread_mutexif (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);
  pthread_mutex;
    if (thrd_success != mtx_unlock(&(queue->mutex));

) {
      return data;
}

In the above code, pthread_mutex_lock() is used to lock the queue.

...

NULL;  /* Indicate failure */
    }
  }
  return data;
}

Risk Assessment

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

Bibliography

Hazard Pointers : Michael, M.M. "Hazard pointers: Safe memory reclamation for lock-free objects,"IEEE Transactions on Parallel and Distributed Systems, vol. 15, no. 8, Aug. 2004.

Linux v2.5.53 : lock-free ipv4 route cache

AMD Developer Central: Articles & Whitepapers:

Lock-Free Programming on AMD Multi-Core System (http://developer.amd.com/documentation/articles/pages/125200689.aspx)

Automated Detection

Related Vulnerabilities

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

Bibliography

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Image Added Image Added Image Added----
CON37-C. Do not use more than one mutex for concurrent waiting operations on a condition variable 14. Concurrency (CON) 49. Miscellaneous (MSC)