Re: [PATCH] ipc/msg: add msgsnd_timed and msgrcv_timed syscall for system V message queue

[Manfred Spraul]

Hi Eric,


On 3/4/21 2:12 AM, Andrew Morton wrote:
> On Tue, 23 Feb 2021 23:11:43 +0800 Eric Gao <eric.tech@xxxxxxxxxxx> wrote:
>
> > sometimes, we need the msgsnd or msgrcv syscall can return after a limited
> > time, so that the business thread do not be blocked here all the time. In
> > this case, I add the msgsnd_timed and msgrcv_timed syscall that with time
> > parameter, which has a unit of ms.
> Please cc Manfred and Davidlohr on ipc/ changes.
>
> The above is a very brief description for a new syscall!  Please go to
> great lengths to tell us why this is considered useful - what are the
> use cases?
>
> Also, please fully describe the proposed syscall interface right here
> in the changelog.  Please be prepared to later prepare a full manpage.
>
> > ...
> > +SYSCALL_DEFINE5(msgsnd_timed, int, msqid, struct msgbuf __user *, msgp, size_t, msgsz,
> > +		int, msgflg, long, timeoutms)
> Specifying the timeout in milliseconds is problematic - it's very
> coarse.  See sys_epoll_pwait2()'s use of timespecs.

What about using an absolute timeout, like in mq_timedsend()?

That makes restart handling after signals far simpler.

> > -               schedule();
> > +
> > +               /* sometimes, we need msgsnd syscall return after a given time */
> > +               if (timeoutms <= 0) {
> > +                       schedule();
> > +               } else {
> > +                       timeoutms = schedule_timeout(timeoutms);
> > +                       if (timeoutms == 0)
> > +                               timeoutflag = true;
> > +               }
>
> I wonder if this should be schedule_timeout_interruptible() or at least
> schedule_timeout_killable() instead of schedule_timeout(). If it should,
> this should probably be done as a separate change.
No. schedule_timeout_interruptible() just means that 
__set_current_state() is called before the schedule_timeout().

The __set_current_state() is done directly in msg.c, before dropping the 
lock.

--

    Manfred