cygwin source-patch fixing deadlock while writing to serial port

[H. Henning Schmidt]

 > > > > 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.

ok. did this. it works. thanks a lot for the tip.
With this change, my deadlock-prevention (which I still left in there, 
paranoyed as I am) just never comes into action.
I attach my modified patch to this message for reference (to be applied 
to the original 1.5.5-1 sources)

;Henning


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