前边三节，我们讲了消息队列的创建、发送信息和接收消息，今天继续沿着应用程序路线看内核中的对消息队列的控制，首先是我们看一下应用程序中的界面函数
msgctl(msgid,IPC_RMID,0)；
通过sys_ipc()系统调用中看到这样一句switch语句代码：
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case MSGCTL:
return sys_msgctl (first, second, (struct msqid_ds __user *) ptr);
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可以看到是进入了sys_msgctl()函数并从此返回，我们追踪一下这个函数
第一参数不言而喻，朋友们通过前三节的阅读这里应该知道是消息队列的ID号，第二个参数我们需要说明一下，应用程序传递过来的是IPC_RMID
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asmlinkage long sys_msgctl(int msqid, int cmd, struct msqid_ds __user *buf)
{
struct msg_queue *msq;
int err, version;
struct ipc_namespace *ns;
if (msqid < 0 || cmd < 0)
return -EINVAL;
version = ipc_parse_version(&cmd);
ns = current->nsproxy->ipc_ns;
switch (cmd) {
case IPC_INFO:
case MSG_INFO:
{
struct msginfo msginfo;
int max_id;
if (!buf)
return -EFAULT;
/*
* We must not return kernel stack data.
* due to padding, it's not enough
* to set all member fields.
*/
err = security_msg_queue_msgctl(NULL, cmd);
if (err)
return err;
memset(&msginfo, 0, sizeof(msginfo));
msginfo.msgmni = ns->msg_ctlmni;
msginfo.msgmax = ns->msg_ctlmax;
msginfo.msgmnb = ns->msg_ctlmnb;
msginfo.msgssz = MSGSSZ;
msginfo.msgseg = MSGSEG;
down_read(&msg_ids(ns).rw_mutex);
if (cmd == MSG_INFO) {
msginfo.msgpool = msg_ids(ns).in_use;
msginfo.msgmap = atomic_read(&ns->msg_hdrs);
msginfo.msgtql = atomic_read(&ns->msg_bytes);
} else {
msginfo.msgmap = MSGMAP;
msginfo.msgpool = MSGPOOL;
msginfo.msgtql = MSGTQL;
}
max_id = ipc_get_maxid(&msg_ids(ns));
up_read(&msg_ids(ns).rw_mutex);
if (copy_to_user(buf, &msginfo, sizeof(struct msginfo)))
return -EFAULT;
return (max_id < 0) ? 0 : max_id;
}
case MSG_STAT: /* msqid is an index rather than a msg queue id */
case IPC_STAT:
{
struct msqid64_ds tbuf;
int success_return;
if (!buf)
return -EFAULT;
if (cmd == MSG_STAT) {
msq = msg_lock(ns, msqid);
if (IS_ERR(msq))
return PTR_ERR(msq);
success_return = msq->q_perm.id;
} else {
msq = msg_lock_check(ns, msqid);
if (IS_ERR(msq))
return PTR_ERR(msq);
success_return = 0;
}
err = -EACCES;
if (ipcperms(&msq->q_perm, S_IRUGO))
goto out_unlock;
err = security_msg_queue_msgctl(msq, cmd);
if (err)
goto out_unlock;
memset(&tbuf, 0, sizeof(tbuf));
kernel_to_ipc64_perm(&msq->q_perm, &tbuf.msg_perm);
tbuf.msg_stime = msq->q_stime;
tbuf.msg_rtime = msq->q_rtime;
tbuf.msg_ctime = msq->q_ctime;
tbuf.msg_cbytes = msq->q_cbytes;
tbuf.msg_qnum = msq->q_qnum;
tbuf.msg_qbytes = msq->q_qbytes;
tbuf.msg_lspid = msq->q_lspid;
tbuf.msg_lrpid = msq->q_lrpid;
msg_unlock(msq);
if (copy_msqid_to_user(buf, &tbuf, version))
return -EFAULT;
return success_return;
}
case IPC_SET:
case IPC_RMID:
err = msgctl_down(ns, msqid, cmd, buf, version);
return err;
default:
return -EINVAL;
}
out_unlock:
msg_unlock(msq);
return err;
}
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在分析这段代码之前我们要看一下参数cmd
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/*
* Control commands used with semctl, msgctl and shmctl
* see also specific commands in sem.h, msg.h and shm.h
*/
#define IPC_RMID 0 /* remove resource */
#define IPC_SET 1 /* set ipc_perm options */
#define IPC_STAT 2 /* get ipc_perm options */
#define IPC_INFO 3 /* see ipcs */
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这些命令代码不仅是为消息队列所使用的，还为整个IPC通讯所使用，包括信号量和共享内存，后边二种我们以后进行实践和分析，另外，也还有消息队列专用的命令
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/* ipcs ctl commands */
#define MSG_STAT 11
#define MSG_INFO 12
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我们还是那种方式在代码中去理解这些宏的作用，不过聪明的读者可能从定义的字面上能看出其作用，为了避免死记硬背，死搬硬套的学习方法，我们就从实践中记忆吧
这里还有第三个参数struct msqid_ds ，它是为了保持兼容以前的代码所用的，我们暂且放一放，避免跑了主题，可以看到在我们的应用程序传递下来的这个参数是空指针。
好了我们进入函数内部看一下，因为使用的命令代码是IPC＿RMID，所以会进入这段case语句
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case IPC_RMID:
err = msgctl_down(ns, msqid, cmd, buf, version);
return err;
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static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
struct msqid_ds __user *buf, int version)
{
struct kern_ipc_perm *ipcp;
struct msqid64_ds msqid64;
struct msg_queue *msq;
int err;
if (cmd == IPC_SET) {
if (copy_msqid_from_user(&msqid64, buf, version))
return -EFAULT;
}
ipcp = ipcctl_pre_down(&msg_ids(ns), msqid, cmd,
&msqid64.msg_perm, msqid64.msg_qbytes);
if (IS_ERR(ipcp))
return PTR_ERR(ipcp);
msq = container_of(ipcp, struct msg_queue, q_perm);
err = security_msg_queue_msgctl(msq, cmd);
if (err)
goto out_unlock;
switch (cmd) {
case IPC_RMID:
freeque(ns, ipcp);
goto out_up;
case IPC_SET:
if (msqid64.msg_qbytes > ns->msg_ctlmnb &&
!capable(CAP_SYS_RESOURCE)) {
err = -EPERM;
goto out_unlock;
}
msq->q_qbytes = msqid64.msg_qbytes;
ipc_update_perm(&msqid64.msg_perm, ipcp);
msq->q_ctime = get_seconds();
/* sleeping receivers might be excluded by
* stricter permissions.
*/
expunge_all(msq, -EAGAIN);
/* sleeping senders might be able to send
* due to a larger queue size.
*/
ss_wakeup(&msq->q_senders, 0);
break;
default:
err = -EINVAL;
}
out_unlock:
msg_unlock(msq);
out_up:
up_write(&msg_ids(ns).rw_mutex);
return err;
}
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上面这段代码的重要作用就是删除消息队列，清除消息，我们重点看里面的几个调用的函数，其余的代码很简单，有基础的朋友可以看的懂，抓住重点来看，直接进入
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static void freeque(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
{
struct list_head *tmp;
struct msg_queue *msq = container_of(ipcp, struct msg_queue, q_perm);
expunge_all(msq, -EIDRM);
ss_wakeup(&msq->q_senders, 1);
msg_rmid(ns, msq);
msg_unlock(msq);
tmp = msq->q_messages.next;
while (tmp != &msq->q_messages) {
struct msg_msg *msg = list_entry(tmp, struct msg_msg, m_list);
tmp = tmp->next;
atomic_dec(&ns->msg_hdrs);
free_msg(msg);
}
atomic_sub(msq->q_cbytes, &ns->msg_bytes);
security_msg_queue_free(msq);
ipc_rcu_putref(msq);
}
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首先是expunge_all()函数
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static void expunge_all(struct msg_queue *msq, int res)
{
struct list_head *tmp;
tmp = msq->q_receivers.next;
while (tmp != &msq->q_receivers) {
struct msg_receiver *msr;
msr = list_entry(tmp, struct msg_receiver, r_list);
tmp = tmp->next;
msr->r_msg = NULL;
wake_up_process(msr->r_tsk);
smp_mb();
msr->r_msg = ERR_PTR(res);
}
}
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expunge_all是擦去的意思，这个函数的的作用就是循环从消息队列中的等待接收的进程开始，依次唤醒他们让他们返回，也就是消息的主体清场开始了，等待消息的客户请回吧。里面的smp与多处理器smp机制有关，这里不用关心，以后我们会接触它。然后执行下一个函数
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static void ss_wakeup(struct list_head *h, int kill)
{
struct list_head *tmp;
tmp = h->next;
while (tmp != h) {
struct msg_sender *mss;
mss = list_entry(tmp, struct msg_sender, list);
tmp = tmp->next;
if (kill)
mss->list.next = NULL;
wake_up_process(mss->tsk);
}
}
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清除所有在消除队列中的等待发送的进程，唤醒他们清场。下面执行
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static inline void msg_rmid(struct ipc_namespace *ns, struct msg_queue *s)
{
ipc_rmid(&msg_ids(ns), &s->q_perm);
}
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void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
{
int lid = ipcid_to_idx(ipcp->id);
idr_remove(&ids->ipcs_idr, lid);
ids->in_use--;
ipcp->deleted = 1;
return;
}
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这里重点的函数就是idr_remove
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void idr_remove(struct idr *idp, int id)
{
struct idr_layer *p;
/* Mask off upper bits we don't use for the search. */
id &= MAX_ID_MASK;
sub_remove(idp, (idp->layers - 1) * IDR_BITS, id);
if (idp->top && idp->top->count == 1 && (idp->layers > 1) &&
idp->top->ary[0]) { // We can drop a layer
p = idp->top->ary[0];
idp->top->bitmap = idp->top->count = 0;
free_layer(idp, idp->top);
idp->top = p;
--idp->layers;
}
while (idp->id_free_cnt >= IDR_FREE_MAX) {
p = alloc_layer(idp);
kmem_cache_free(idr_layer_cache, p);
}
return;
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我们看到他释放了我们以前建立消息队列时分配的idr_layer结构，这里我们需要提醒一下，删除队列时唤醒等待接收和发送的进程他们将要执行什么路线，我们回忆一下前二节的内容，接收和发送的进程在唤醒后也就是从shedule()返回到他们各自的代码中时，就会再次对所在的消息队列加锁，如果加锁失败他们此时就会退出接收和发送代码部分退出了，关于加锁rcu机制我们以后重点在讲锁的部分进行介绍，至于其余的代码都很简单了，朋友们可以根据这条主线举一反三的阅读一下。下一节我们探讨共享内存的实践和内核。
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