Re: How do you kill a completely locked up thread?

"Jeroen Mostert" wrote:
I'm not an MSVP but I've seen this so many times in our codebase that it's
not funny anymore. The one way to cancel pending I/O on a socket and unwedge
threads blocking on that is to close the socket from another thread and
handle the resulting exceptions. Nothing else will do, at least nothing that
can be called reliable. Of course, this means tearing down the connection,
but that's still a whole lot better than tearing down your process.

The other alternative, which is less straightforward but suits some designs
better, is to make sure that threads never issue I/O which can take
"forever". Almost every I/O call has a timeout parameter, and for those that
don't there's always asynchronous I/O and
ThreadPool.RegisterWaitForSingleObject(). When the call returns with a
timeout, either poll, decide to wait some more or give up and close the
socket, which you can then do from the same thread that owns the socket,
simplifying error handling.

I understand it's not your code, but trust me: you'll want to rewrite it
anyway, unless you can afford restarting your application every so often.

Grrr.. Damm post thing asked me to login again and ate me post... Anyway...

The SharpSSH code base has a bunch of classes and would take a major effort
to re-write. It is clear that it is unfinished from looking at it. I am
not sure my company wants to fund me re-writing this code set.

However, the solution Peter Bromberg gave on his web site looks good except
for what appears to me to be a big hole or leak. I do not understand how C#
handles this, so maybe it is a non issue. The following is the code segment
from his web site, I hope he doesn't mind me posting it:

public ArrayList DoWorkNeedsTimeout(ArrayList alin, int secondsToWait)
{

ArrayList alOut = new ArrayList();

//Create an instance of our delegate, pointing to the helper
method:

DoWorkNeedsTimeoutDelegate deleg = new
DoWorkNeedsTimeoutDelegate(DoWorkWithTimeout);

// Call BeginInvoke on delegate.
// Note on last two parameters of Delegate BeginInvoke Method:
// 1) callback: not used here, we can pass null
// 2) state: not used, pass an instance of object in the required
parameter location
// Invoke the delegate passing the parameters and get the
IAsyncResult object in "ar":

IAsyncResult ar = deleg.BeginInvoke(alin, secondsToWait, null,
new object());

// if the WaitOne method times out before we get a result, it
will be false:
if (!ar.AsyncWaitHandle.WaitOne(5000, false))
{

// handle timeout logging / notification here - Syslog,
Database, Email - whatever you need
alOut.Add("TIMED OUT!");
}

else // we didn't time out:
{
// get the result of the method call here
alOut = deleg.EndInvoke(ar);
}

return alOut;

}

What he is doing is making a delegate to call BeginInvoke with and then
using the IAsyncResult to wait for a time peroid. If the time peroid
expires, then his thread continues on. If it doesn't expire, he calls
EndInvoke(). This looks good except for the issue of dealing with a truely
locked-up thread.

BeginInvoke() uses a thread from the thread-pool right? So what happens if
that thread never returns so you can call End-Invoke? Is it gone from the
thread-pool forever? If you repeat this look 1000s of times and even if 1%
of the time you get a locked up thread, won't you run out of threads?

The only way this can work indefinitly, which it may, is if the Garbage
collector will reclaim the thread once the delegate and other related objects
are out of scope. Is this how it works?

.

Relevant Pages

  • Re: How do you kill a completely locked up thread?
    ... The one way to cancel pending I/O on a socket and unwedge threads blocking on that is to close the socket from another thread and handle the resulting exceptions. ... // Call BeginInvoke on delegate. ... // Note on last two parameters of Delegate BeginInvoke Method: ...
    (microsoft.public.dotnet.languages.csharp)
  • Looking for advice on a robust TCP/IP client implementation
    ... exception is encountered. ... additional data needs to be sent back via the socket, ... /// Event delegate to use for the connection and disconnection ... public delegate void ConnectionEventHandler(object sender, ...
    (microsoft.public.dotnet.framework.compactframework)
  • Re: How do you kill a completly locked up thread?
    ... This seems to be the only C# method I know of to kill a thread. ... For one thing, you can kill off the thread corresponding to the Thread object, but this is not guaranteed to be the thread doing the actual blocking I/O, it might just be waiting on another thread. ... When the thread is doing this connect (it will happen with even a simple TCP/IP socket connect) issue a Thread.Abortfrom another thread and you will see that the ThreadAbortException will NOT be thrown until the Connect call returns. ...
    (microsoft.public.dotnet.languages.csharp)
  • Re: http webserver using select()
    ... "Binding" file descriptor with open file to accepted socket descriptor ... fds for each connection. ... generate socket fd via socket-> listening socket ... For other 1 bits in fd_set, handle I/O ...
    (comp.unix.programmer)
  • Re: java socket i/o with callback/non-blocking
    ... Basically I want to have some subroutine to be called whenever there is data available to read on a socket object without having to poll it. ... Note also that the technique described is blocking, not non-blocking, as indeed is the technique you described even if the channel is in non-blocking mode. ... For just one socket, you are probably much better off to dump all the NIO stuff, and simply use blocking I/O in its own thread. ...
    (comp.lang.java.programmer)