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Date: Mon, 19 Jan 2004 15:37:39 -0600 (CST)
From: Brian Ford <ford AT vss DOT fsi DOT com>
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To: cygwin AT cygwin DOT com
Subject: Re: cygwin source-patch fixing deadlock while writing to serial port
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On Mon, 19 Jan 2004, Christopher Faylor 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.

> >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.

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

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