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Re: Shells hang during script execution
- From: "Ernie Coskrey" <Ernie dot Coskrey at steeleye dot com>
- To: <cygwin at cygwin dot com>
- Date: Thu, 16 Mar 2006 15:14:03 -0500
- Subject: 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 pipe.cc 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
>>>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 cygthread.cc.udiff
>>--- cygthread.cc.ORIG 2006-02-22 10:57:42.123931300 -0500
>>+++ cygthread.cc 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.
--- cygthread.cc.ORIG 2006-03-01 17:40:44.000000000 -0500
+++ cygthread.cc 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);
- 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 @@
- if (ev)
- ResetEvent (ev);
@@ -223,7 +221,6 @@
WaitForSingleObject (ev, INFINITE);
- ResetEvent (ev);
h = htobe;
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