c++ - Why should Next() operation of a singly linked list be protected with a critical section? -

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i'm reading book multithreading applications in win32

the book says return node->next compiled separate machine instructions not executed atomic operation, next() should protected critical section.

my question is, instructions translated into, cause race condition?

typedef struct _node {     struct node *next;     int data; } node;  typedef struct _list {     node *head;     critical section critical_sec; } list;  list *createlist() {     list *plist = malloc(sizeof(list));     plist->head = null;     initializecriticalsection(&plist->critical_sec);     return plist; }  void deletelist(list *plist) {     deletecriticalsection(&plist->critical_sec);     free(plist); }  void addhead(list *plist, node *node) {     entercriticalsection(&plist->critical_sec);     node->next = plist->head;     plist->head = node;     leavecriticalsection(&plist->critical_sec); }  void insert(list *plist, node *afternode, node *newnode) {     entercriticalsection(&plist->critical_sec);     if (afternode == null)     {         addhead(plist, newnode);     }     else     {         newnode->next = afternode->next;         afternode->next = newnode;     }     leavecriticalsection(&plist->critical_sec); }  node *next(list *plist, node *node) {     node* next;     entercriticalsection(&plist->critical_sec);     next = node->next;     leavecriticalsection(&plist->critical_sec);     return next; }

edit:

ok, although in particular case won't corrupt singly linked list w/o protecting next() operation, shared structure should protected whole or nothing, generally.

return node->next performs 2 operations; first loads struct pointed node memory, looks @ node+offsetof(next) find pointer next, load register, , return calling program. contents of node may manipulated thread of execution in meantime.


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