On Thu, Sep 1, 2016 at 12:38 PM, Richard Braun <rbr...@sceen.net> wrote:
> > > Most modern microkernels use synchronous IPC
> > > that just block, and many operations on top of them do block. The
> > > overall system on the other hand must be careful to avoid such
> > > deadlocks.
> >
> > OK, I read the Mach documentation for mach_msg() and concluded that it
> was
> > like a POSIX read(), that I could operate it in a mode where the kernel
> > absolutely would not block my process, and would return EWOULDBLOCK
> > instead. That's basically a kernel guarantee, at least as much as it is.
> > (Notice that it doesn't guarantee how long the system call will take - 1
> > ms? 1 s? 1 week? - because it's not a real time system, which is why I
> > say "as much as it is")
>
> Yes, you can think of mach_msg as such a system call. Note that if
> the timeout is 0, it will return immediately instead of blocking,
> which is what a real-time system would do too. Real time systems
> aren't about that at all.
>
> > Are you now saying that's not how it works on Mach/Hurd? If so, please
> let
> > me know, because I've been under a big misunderstanding that I need to
> get
> > cleared up!
>
> I think your mistake here is using MACH_SEND_TIMEOUT instead of
> MACH_RCV_TIMEOUT. Your message certainly was sent, so there is no
> reason to get a timeout error for that.
>
Here's the call:
mach_msg(msg, MACH_SEND_MSG | MACH_SEND_TIMEOUT, msg->msgh_size,
0, msg->msgh_remote_port,
0, MACH_PORT_NULL);
Why should I specify MACH_RCV_TIMEOUT? It's not a receive call.
The way my code is structured, one thread handles the traffic from the
network to IPC, and a separate thread handles the traffic from IPC to the
network. This call is in the network-to-IPC thread. This thread never
receives anything except network traffic, which it blocks for.
I want a non-blocking send. This one blocks if the message is vm_map, the
memory object passed in is goofed, and the message is targeted at a task
port on the local kernel (and it doesn't have to be the task that calls
mach_msg).
> Now that you know you should be using MACH_RCV_TIMEOUT, you should see
> that no server can block you indefinitely.
Just so we're on the same page here, should that call above block or not?
I just tried it again with MACH_RCV_TIMEOUT; it does the same thing.
agape
brent
