c - Mutual exclusion isn't exclusive -

i have following code runs in 2 threads started init call main thread. 1 writing device, 1 reading. app called other threads add items queues. pop_queue handles locking, push_queue. whenever modify req r, lock it's mutex. q->process function pointer 1 of either write_sector, read_setor. need guard against simultaneous calls 2 function pointers, i'm using mutex on actual process call, not working.

according text program, making parallel calls process functions. how possible given lock immediatly before , unlock afterwards?

the following error valgrind --tool=helgrind might help?

==3850== possible data race during read of size 4 @ 0xbea57efc thread #2 ==3850==    @ 0x804a290: request_handler (diskdriver.c:239) 

line 239 r->state = q->process(*device, &r->sd) +1

void * request_handler(void *arg) {     req *r;     queue *q = arg;     int writing = !strcmp(q->name, "write");     for(;;) {         /*          * wait request          */         pop_queue(q, &r, true);          /*          * handle request          * req r unattached lists, must lock it's properties incase being redeemed          */         printf("info: driver: (%s) handling req %d\n", q->name, r->id);         pthread_mutex_lock(&r->lock);          pthread_mutex_lock(&q->processing);         r->state = q->process(*device, &r->sd) +1;         pthread_mutex_unlock(&q->processing);          /*           * if writing, return sectordescriptor          */          if (writing) {             printf("info: driver (write thread) has released sector descriptor.\n");             blocking_put_sd(*sd_store, r->sd);             r->sd = null;         }          pthread_mutex_unlock(&r->lock);         pthread_cond_signal(&r->changed);      } } 

edit

here 1 other location req's properties read

int redeem_voucher(voucher v, sectordescriptor *sd) {     int result;      if (v == null){         printf("driver: null voucher redeemed!\n");         return 0;     }     req *r = v;     pthread_mutex_lock(&r->lock);      /* if state = 0 job still running/queued */     while(r->state==0) {         printf("driver: blocking req %d finish\n", r->id);         pthread_cond_wait(&r->changed, &r->lock);     }      sd = &r->sd;     result = r->state-1;     r->sd = null;     r->state = wait;     //printf("driver: req %d completed\n", r->id);     pthread_mutex_unlock(&r->lock);     /*      * return req pool      */         push_queue(&pool_q, r);     return result; } 

edit 2 here's push_ , pop_queue functions

int pop_queue(struct queue *q, req **r, int block) {     pthread_mutex_lock(&q->lock);     while(q->head == null) {         if(block) {             pthread_cond_wait(&q->wait, &q->lock);         }         else {             pthread_mutex_unlock(&q->lock);             return false;         }     }      req *got = q->head;     q->head = got->next;     got->next = null;     if(!q->head) {         /* removed last element */         q->tail = q->head;     }      *r = got;     pthread_mutex_unlock(&q->lock);     return true; }  /*  * perform standard linked list insertion queue specified  * handles required locking , signals listeners  * return: int - if insertion successful  */ int push_queue(queue *q, req *r) {     /*      * push never blocks,       */     if(!r || !q)         return false;      pthread_mutex_lock(&q->lock);      if(q->tail) {         q->tail->next = r;         q->tail = r;     }     else {         /* empty queue */         q->tail = q->head = r;     }      pthread_mutex_unlock(&q->lock);     pthread_cond_signal(&q->wait);      return true; } 

based on available information, seems possibility thread modifying data pointed *device. perhaps being modified while q->processing mutex not held.