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Date: Mon, 19 Jan 2004 16:14:40 -0500
From: Christopher Faylor <cgf-no-personal-reply-please AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: cygwin source-patch fixing deadlock while writing to serial port
Message-ID: <20040119211440.GG19903@redhat.com>
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References: <400C4757 DOT 3060208 AT hhschmidt DOT de>
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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;
>
>The applied patch adds a safety exit to that for(;;) loop.
>This fixes the testcase referenced above.
>
>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.

cgf

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