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Re: Shells hang during script execution

>On Wed, Mar 01, 2006 at 01:01:46PM -0500, Ernie Coskrey wrote:
>>>>Here's a description of a second hang condition we were encountering, along 
>>>>with a patch for it.
>>>>The application (pdksh in this case) does a read on a pipe, which eventually 
>>>>calls fhandler_pipe::read in Thread 1.  This creates a new cygthread 
>>>>with "read_pipe()" as the function.  Then >it calls th->detach(read_state).
>>>>When the hang occurs, the new thread gets terminated early, before
>>>>cygthread::stub() can call "callfunc()".  You see the error message
>>>>"erroneous thread activation".  I'm not sure what's causing the thread
>>>>to fail activation, but the result is, the read_state semaphore never
>>>>gets signalled.
>>>Sorry but this is another band-aid around a problem.  The real problem
>>>is that the code shouldn't get into the state that you are describing.
>>>That's why cygwin prints an error message - it is a serious problem.
>>>Making the code deal gracefully with a problem like this isn't going
>>>to solve the underlying issue.
>>>If you can figure out what's causing the erroneous thread activation
>>>then that will be the real culprit.
>>OK, I believe I've tracked this down.
>>The problem occurs when we get into a read_pipe cygthread constructor
>>(cygthread::cygthread()) with a NULL h and an ev that is signalled.
>>When this condition exists, a hang can occur as follows:
>>1) Creator thread calls detach().  This waits for pipe_state to be released twice
>>2) read_pipe thread calls read_pipe, reads data, and releases the semaphore twice
>>3) Creator thread goes to WFSO(*this, INFINITE) which returns immediately because ev was set when the thread was created.
>>4) Creator thread initiates another read_pipe cygthread to read more pipe data.
>>At this point, there's a race: if the Creator thread gets past the
>>initialization part of the constuctor, which sets __name(name), BEFORE
>>the original read_pipe thread gets to the part of cygthread::stub()
>>that sets info->__name = NULL, then you'll see the hang.  The new
>>pipe_read will give the "erroneous thread activation" message, and the
>>parent will be stuck waiting for data that will never arrive.
>>The only path that leaves an unused thread structure in a state where
>>h==NULL and ev is signalled is cygthread::release().  So the fix is
>>$ cat
>>---   2006-02-22 10:57:42.123931300 -0500
>>+++        2006-03-01 12:59:23.255023000 -0500
>>@@ -268,7 +268,12 @@
>> cygthread::release (bool nuke_h)
>> {
>>   if (nuke_h)
>>+    {
>>     h = NULL;
>>+    if (ev)
>>+      ResetEvent (ev);
>>+    }
>> #ifdef DEBUGGING
>>   __oldname = __name;
>>   debug_printf ("released thread '%s'", __oldname);
>Nice analysis.  Thank you.  I think it's easier to fix this by just
>making the ev event auto-reset then this condition would be caught in
>terminate thread, as it was meant to be.

Here's a patch for the problem that works with the latest snapshot.

Ernie Coskrey       SteelEye Technology, Inc.

---	2006-03-01 17:40:44.000000000 -0500
+++	2006-03-16 14:54:04.148312500 -0500
@@ -78,7 +78,7 @@
       debug_printf ("thread '%s', id %p, stack_ptr %p", info->name (), info->id, info->stack_ptr);
       if (!info->ev)
-	  info->ev = CreateEvent (&sec_none_nih, TRUE, FALSE, NULL);
+	  info->ev = CreateEvent (&sec_none_nih, FALSE, FALSE, NULL);
 	  info->thread_sync = CreateEvent (&sec_none_nih, FALSE, FALSE, NULL);
@@ -197,8 +197,6 @@
   HANDLE htobe;
   if (h)
-      if (ev)
-	ResetEvent (ev);
       while (!thread_sync)
 	low_priority_sleep (0);
       SetEvent (thread_sync);
@@ -223,7 +221,6 @@
       while (!ev)
 	low_priority_sleep (0);
       WaitForSingleObject (ev, INFINITE);
-      ResetEvent (ev);
   h = htobe;

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