IPC-消息队列

struct msgbuf {  long mtype;//消息类型  char mtext[1024];//消息数据};

// 先看msgsnd()函数，它通过系统调用接口界面，进入内核执行，代码如下：SYSCALL_DEFINE4(msgsnd, int, msqid, struct msgbuf __user *, msgp, size_t, msgsz,        int, msgflg){    long mtype;     if (get_user(mtype, &msgp->mtype))        return -EFAULT;    return do_msgsnd(msqid, mtype, msgp->mtext, msgsz, msgflg);}// 接下来看do_msgsnd()部分的代码，如下：long do_msgsnd(int msqid, long mtype, void __user *mtext,        size_t msgsz, int msgflg){    struct msg_queue *msq;    struct msg_msg *msg;    int err;    struct ipc_namespace *ns;     ns = current->nsproxy->ipc_ns;     if (msgsz > ns->msg_ctlmax || (long) msgsz < 0 || msqid < 0)        return -EINVAL;    if (mtype < 1)        return -EINVAL;     msg = load_msg(mtext, msgsz);    if (IS_ERR(msg))        return PTR_ERR(msg);     msg->m_type = mtype;    msg->m_ts = msgsz;     msq = msg_lock_check(ns, msqid);    if (IS_ERR(msq)) {        err = PTR_ERR(msq);        goto out_free;    }     for (;;) {        struct msg_sender s;         err = -EACCES;        if (ipcperms(&msq->q_perm, S_IWUGO))            goto out_unlock_free;         err = security_msg_queue_msgsnd(msq, msg, msgflg);        if (err)            goto out_unlock_free;         if (msgsz + msq->q_cbytes <= msq->q_qbytes &&                1 + msq->q_qnum <= msq->q_qbytes) {            break;        }         /* queue full, wait: */        if (msgflg & IPC_NOWAIT) {            err = -EAGAIN;            goto out_unlock_free;        }        ss_add(msq, &s);        ipc_rcu_getref(msq);        msg_unlock(msq);        schedule();         ipc_lock_by_ptr(&msq->q_perm);        ipc_rcu_putref(msq);        if (msq->q_perm.deleted) {            err = -EIDRM;            goto out_unlock_free;        }        ss_del(&s);         if (signal_pending(current)) {            err = -ERESTARTNOHAND;            goto out_unlock_free;        }    }     msq->q_lspid = task_tgid_vnr(current);    msq->q_stime = get_seconds();     if (!pipelined_send(msq, msg)) {        /* noone is waiting for this message, enqueue it */        list_add_tail(&msg->m_list, &msq->q_messages);        msq->q_cbytes += msgsz;        msq->q_qnum++;        atomic_add(msgsz, &ns->msg_bytes);        atomic_inc(&ns->msg_hdrs);    }     err = 0;    msg = NULL; out_unlock_free:    msg_unlock(msq);out_free:    if (msg != NULL)        free_msg(msg);    return err;}// 在这段代码中，请注意临近入口位置的这个函数msg_lock_check()，我们跟进，看一下这个lock是如何check// 的，代码如下：static inline struct msg_queue *msg_lock_check(struct ipc_namespace *ns,                        int id){    struct kern_ipc_perm *ipcp = ipc_lock_check(&msg_ids(ns), id);     if (IS_ERR(ipcp))        return (struct msg_queue *)ipcp;     return container_of(ipcp, struct msg_queue, q_perm);}// ipc_lock_check()是一个能够check所有IPC object同步信息的函数，它的定义如下：struct kern_ipc_perm *ipc_lock_check(struct ipc_ids *ids, int id){    struct kern_ipc_perm *out;     out = ipc_lock(ids, id);    if (IS_ERR(out))        return out;     if (ipc_checkid(out, id)) {        ipc_unlock(out);        return ERR_PTR(-EIDRM);    }     return out;}// 这里的ipc_lock()是至关重要的地方！通过这个函数的注释，也能明白它的作用了：/** * ipc_lock - Lock an ipc structure without rw_mutex held * @ids: IPC identifier set * @id: ipc id to look for * * Look for an id in the ipc ids idr and lock the associated ipc object. * * The ipc object is locked on exit. */ struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id){    struct kern_ipc_perm *out;    int lid = ipcid_to_idx(id);     rcu_read_lock();    out = idr_find(&ids->ipcs_idr, lid);    if (out == NULL) {        rcu_read_unlock();        return ERR_PTR(-EINVAL);    }     spin_lock(&out->lock);         /* ipc_rmid() may have already freed the ID while ipc_lock     * was spinning: here verify that the structure is still valid     */    if (out->deleted) {        spin_unlock(&out->lock);        rcu_read_unlock();        return ERR_PTR(-EINVAL);    }     return out;}