[Devel] Re: [PATCH v8 4/5] ipc: message queue copy feature introduced

[Andrew Morton]

On Wed, 24 Oct 2012 19:35:20 +0400
Stanislav Kinsbursky <skinsbursky at parallels.com> wrote:

> This patch is required for checkpoint/restore in userspace.
> IOW, c/r requires some way to get all pending IPC messages without deleting
> them from the queue (checkpoint can fail and in this case tasks will be resumed,
> so queue have to be valid).
> To achive this, new operation flag MSG_COPY for sys_msgrcv() system call was
> introduced. If this flag was specified, then mtype is interpreted as number of
> the message to copy.
> If MSG_COPY is set, then kernel will allocate dummy message with passed size,
> and then use new copy_msg() helper function to copy desired message (instead of
> unlinking it from the queue).
> 
> ...
>
> @@ -777,19 +777,48 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>  	struct msg_msg *msg;
>  	int mode;
>  	struct ipc_namespace *ns;
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +	struct msg_msg *copy = NULL;
> +	unsigned long copy_number = 0;
> +#endif
>  
>  	if (msqid < 0 || (long) bufsz < 0)
>  		return -EINVAL;
> +	if (msgflg & MSG_COPY) {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +
> +		if (msgflg & MSG_COPY) {

This test is't needed.

> +			copy_number = msgtyp;
> +			msgtyp = 0;
> +		}
> +
> +		/*
> +		 * Create dummy message to copy real message to.
> +		 */
> +		copy = load_msg(buf, bufsz);
> +		if (IS_ERR(copy))
> +			return PTR_ERR(copy);
> +		copy->m_ts = bufsz;
> +#else
> +		return -ENOSYS;
> +#endif
> +	}
>  	mode = convert_mode(&msgtyp, msgflg);
>  	ns = current->nsproxy->ipc_ns;
>  
>  	msq = msg_lock_check(ns, msqid);
> -	if (IS_ERR(msq))
> +	if (IS_ERR(msq)) {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +		if (msgflg & MSG_COPY)
> +			free_msg(copy);
> +#endif
>  		return PTR_ERR(msq);
> +	}
>  
>  	for (;;) {
>  		struct msg_receiver msr_d;
>  		struct list_head *tmp;
> +		long msg_counter = 0;
>  
>  		msg = ERR_PTR(-EACCES);
>  		if (ipcperms(ns, &msq->q_perm, S_IRUGO))
> @@ -809,8 +838,16 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>  				if (mode == SEARCH_LESSEQUAL &&
>  						walk_msg->m_type != 1) {
>  					msgtyp = walk_msg->m_type - 1;
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +				} else if (msgflg & MSG_COPY) {
> +					if (copy_number == msg_counter) {
> +						msg = copy_msg(walk_msg, copy);
> +						break;
> +					}
> +#endif
>  				} else
>  					break;
> +				msg_counter++;
>  			}
>  			tmp = tmp->next;
>  		}
> @@ -823,6 +860,10 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>  				msg = ERR_PTR(-E2BIG);
>  				goto out_unlock;
>  			}
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +			if (msgflg & MSG_COPY)
> +				goto out_unlock;
> +#endif
>  			list_del(&msg->m_list);
>  			msq->q_qnum--;
>  			msq->q_rtime = get_seconds();
> @@ -906,8 +947,13 @@ out_unlock:
>  			break;
>  		}
>  	}
> -	if (IS_ERR(msg))
> +	if (IS_ERR(msg)) {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +		if (msgflg & MSG_COPY)
> +			free_msg(copy);
> +#endif
>  		return PTR_ERR(msg);
> +	}
>  
>  	bufsz = msg_handler(buf, msg, bufsz);
>  	free_msg(msg);

It's all a bit ugly, but I don't really see much we can practically do
about that.

You could add something like

#ifdef CONFIG_CHECKPOINT_RESTORE
static inline void free_copy(void *p, int msgflg, struct msg_msg *copy)
{
	if (IS_ERR(p) && (msgflg & MSG_COPY))
		free_msg(copy);
}
#else
/* As a macro because `copy' will be undefined */
#define free_copy(p, msgflg, copy) do {} while (0)
#endif

and use that in a couple of places.  But that won't help much.