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 -- Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple Problem reports: http://cygwin.com/problems.html Documentation: http://cygwin.com/docs.html FAQ: http://cygwin.com/faq/
