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多个进程，同时读写消息队列，是否会冲突
来源: 互联网 发布时间：2016-10-27
本文导语: 多个进程，同时读写消息队列，是否会冲突。 请问这个多个进程同时写消息队列，是否需要用mutex来进行同步啊。 | OK！来总结一下这个问题，因为我也不确定，所以写的东西未必正确，错误...
多个进程，同时读写消息队列，是否会冲突。
请问这个多个进程同时写消息队列，是否需要用mutex来进行同步啊。
请问这个多个进程同时写消息队列，是否需要用mutex来进行同步啊。
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OK！来总结一下这个问题，因为我也不确定，所以写的东西未必正确，错误的地方，尽管拍砖！
我的内核代码是2.6.29.1。
OK。至此分析完了一个msgsnd()的实现部分，也就是说内核内部已经为我们解决了同步问题了。不需要用户再去care这个问题了。msgrcv()的实现部分也是一样的道理。
我的内核代码是2.6.29.1。
// 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 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 q_qbytes &&
1 + msq->q_qnum 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;
}
OK。至此分析完了一个msgsnd()的实现部分，也就是说内核内部已经为我们解决了同步问题了。不需要用户再去care这个问题了。msgrcv()的实现部分也是一样的道理。
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sun的mq_receive函数的man page：
If more than one process (or thread) is waiting to receive a message when a message arrives at an empty queue, then the process of highest priority that has been waiting the longest is selected to receive the message.
ms_send的man page:
If more than one thread is waiting to send when space becomes available in the message queue, then the thread of the highest priority which has been waiting the longest is unblocked to send its message. Otherwise, it is unspecified which waiting thread is unblocked. If the specified message queue is full and O_NONBLOCK is set in the message queue description associated with mqdes, the message is not queued and mq_send() returns an error.
看起来从应用层面是不用管理冲突的。
If more than one process (or thread) is waiting to receive a message when a message arrives at an empty queue, then the process of highest priority that has been waiting the longest is selected to receive the message.
ms_send的man page:
If more than one thread is waiting to send when space becomes available in the message queue, then the thread of the highest priority which has been waiting the longest is unblocked to send its message. Otherwise, it is unspecified which waiting thread is unblocked. If the specified message queue is full and O_NONBLOCK is set in the message queue description associated with mqdes, the message is not queued and mq_send() returns an error.
看起来从应用层面是不用管理冲突的。
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