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#include 
   
    #include 
    
     #define NHASH 29#define HASH(id) (((unsigned long)id)%NHASH)struct foo *fh[NHASH];pthread_mutex_t hashlock=PTHREAD_MUTEX_INITIALIZER;struct foo{    int f_count;    pthread_mutex_t f_lock;    int f_id;    struct foo *f_next;};struct foo* foo_alloc(int id){    struct foo *fp;    int idx;    if((fp=malloc(sizeof(struct foo)))!=NULL)    {        fp->f_count=1;        fp->f_id=id;        if(pthread_mutex_init(&fp->f_lock, NULL)!=0)        {            free(fp);            return NULL;        }        idx=HASH(id);        pthread_mutex_lock(&hashlock);        fp->f_next=fh[idx];        fh[idx]=fp;        pthread_mutex_lock(&fp->f_lock);        pthread_mutex_unlock(&hashlock);        pthread_mutex_unlock(&fp->f_lock);    }    return fp;}void foo_hold(struct foo *fp){    pthread_mutex_lock(&fp->f_lock);    fp->f_count++;    pthread_mutex_unlock(&fp->f_lock);}struct foo *foo_find(int id){    struct foo *fp;    pthread_mutex_lock(&hashlock)    for(fp=fh[HASH(id)];fp!=NULL;fp=fp->next)    {        if(fp->f_id==id)        {            foo_hold(fp);            break;        }    }    pthread_mutex_unlock(&hashlock);    return fp;}void foo_rele(struct foo *fp){    struct foo *tfp;    int idx;        pthread_mutex_lock(&fp->f_lock);    if(fp->f_count==1)    {        pthread_mutex_unlock(&hashlock);        pthread_mutex_lock(&hashlock);        pthread_mutex_lock(&fp->f_lock);        if(fp->f_count!=1)        {            fp->f_count--;            pthread_mutex_unlock(&fp->f_lock);            pthread_mutex_unlock(&hashlock);            return;        }        idx=HASH(fp->f_id);        tfp=fh[idx];        if(tfp==fp)        {            fh[idx]=fp->f_next;        }        else        {            while(tfp->f_next!=fp)                tfp=tfp->f_next;            tfp->f_next=fp->f_next;        }        pthread_mutex_unlock(&hashlock);        pthread_mutex_unlock(&fp->f_lock);        pthread_mutex_destroy(&fp_>f_lock);        free(fp);    }    else    {        fp->f_count--;        pthread_mutex_unlock(&fp->f_lock);    }    }
    
   

Using two mutexes

#include 
   
    #include 
    
     #define NHASH 29#define HASH(id) (((unsigned long)id)%NHASH)struct foo *fh[NHASH];pthread_mutex_t hashlock=PTHREAD_MUTEX_INITIALIZER;struct foo{    int f_count;    pthread_mutex_t f_lock;    int f_id;    struct foo *f_next;};struct foo* foo_alloc(int id){    struct foo *fp;    int idx;    if((fp=malloc(sizeof(struct foo)))!=NULL)    {        fp->f_count=1;        fp->f_id=id;        if(pthread_mutex_init(&fp->f_lock, NULL)!=0)        {            free(fp);            return NULL;        }        idx=HASH(id);        pthread_mutex_lock(&hashlock);        fp->f_next=fh[idx];        fh[idx]=fp;        pthread_mutex_lock(&fp->f_lock);        pthread_mutex_unlock(&hashlock);        pthread_mutex_unlock(&fp->f_lock);    }    return fp;}void foo_hold(struct foo *fp){    pthread_mutex_lock(&fp->f_lock);    fp->f_count++;    pthread_mutex_unlock(&fp->f_lock);}struct foo *foo_find(int id){    struct foo *fp;    pthread_mutex_lock(&hashlock)    for(fp=fh[HASH(id)];fp!=NULL;fp=fp->next)    {        if(fp->f_id==id)        {            foo_hold(fp);            break;        }    }    pthread_mutex_unlock(&hashlock);    return fp;}void foo_rele(struct foo *fp){    struct foo *tfp;    int idx;        pthread_mutex_lock(&fp->f_lock);    if(--fp->f_count==0)    {        idx=HASH(fp->f_id);        tfp=fh[idx];        if(tfpp==fp)        {            fh[idx]=fp->f_next;        }        else        {            while(tfp->f_next!=fp)                tfp=tfp->f_next;            tfp->f_next=fp->f_next;        }        pthread_mutex_unlock(&hashlock);        pthread_mutex_destroy(&fp->f_lock);        free(fp);    }    else    {        pthread_mutex_unlock(&fp->f_lock);    }    }
    
   

Simplified locking

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