Re: cygwin source-patch fixing deadlock while writing to serial port

[Brian Ford <ford at vss dot fsi dot com>]

On Tue, 20 Jan 2004, H. Henning Schmidt wrote:

> > > On Mon, Jan 19, 2004 at 10:08:39PM +0100, H. Henning Schmidt wrote:
> > > >I found a potential deadlock while writing to a serial port (e.g.
> > > >/dev/com1) that has been opened as O_RDWR. The deadlock occurs from time
> > > >to time (not sure about exact conditions) when I write to that port,
> > > >while there is data coming in (e.g. from an external device) and I do
> > > >not read away that data fast enough from the port.
> > > >
> > > >I did provide a test case a while ago in
> > > >http://sources.redhat.com/ml/cygwin/2003-03/msg01529.html. I digged into
> > > >the issue some more now and found that the executing thread got
> > > >sometimes deadlocked in fhandler_serial::raw_write(). It basically ends
> > > >up in a for(;;) loop and just never hits the break;
> > > >
> > Exactly.  When the input buffer overflows, all serial communications cease
> > and calls exit with ERROR_OPERATION_ABORTED.  If you only call write, then
> > the ClearCommError() necessary to start things up again is never called,
> > and you stick in that infinite loop.
>
> Ok, I will be happy to learn what the proper fix looks like in your opinion.
>
When I can get to that code, I'll post it.

> However, I have a hard time accepting the fact that I cannot write OUT the
> serial port because my INput buffer overflows. At least when I have switched
> off all kinds of flow control (which I have), then I consider these to be two
> independent streams that happen to share one filedesc. Or should I perhaps
> open two independent filedescriptors in this case, one write-only, one
> read-only?
>
> Are you saying that the underlying Win-API enforces what you state above, or does
> this really make sense in some way and I just didn't get it yet? Your statement
> sounds to me like I am forced to keep a second thread around, and if only to get
> me out of that trap, once I get hung there ... does not sound too elegant to me ...
>
Yes.  I was stating the limitations of the Win-API.

> > > >The applied patch adds a safety exit to that for(;;) loop.
> > > >This fixes the testcase referenced above.
> > > >
> > Yuck!  No, this is not the proper fix.
> >
> > > >This might not be the last problem lingering in the serial access code
> > > >(there are some FIXME tokens still around ...), but it is definitely an
> > > >improvement for me. I thought I'd share that with you.
> > >
> > > Can you convince me that this isn't just a band-aid?  I don't understand
> > > why cygwin *shouldn't* hang in a situation like this.  There are
> > > certainly similar situations where this happens on linux.
> > >
> > > Perhaps we need a low_priority_sleep (10) in the loop in that situation
> > > or something.
> > >
> > No.  I have a partial patch for the above, but I am in the process of
> > getting a new Windows box and shuffling all my data.  I'll try to submit
> > it when things settle if no one beats me to it.
>
> who am I to beat you ...
> but be assured that I'll be waiting eagerly for this patch.
> This issue has been kicking me for a long time (letting my app hang up every once in
> a while), and I really want get this fixed. So now that I know that there is a better
> way to do it ... that's what I'm longing for then :-)
>
While you're waiting, why don't you try what I said.  Put a ClearCommError
call in the switch for the error stated.  I bet you get 90+% of what you
want.

-- 
Brian Ford
Senior Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
Phone: 314-551-8460
Fax:   314-551-8444

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