linux-kernel - Re: Possible 2.6.24-rc7 issue w/respect to pthreads

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Date:	Wed, 9 Jan 2008 12:07:22 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	tom@...aga.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Possible 2.6.24-rc7 issue w/respect to pthreads

On Wed, 9 Jan 2008 02:35:32 -0800 (PST)
tom@...aga.com wrote:

> To Whom It May Concern,
> 
> After I patched my 2.6.23 kernel to 2.6.24-rc7 this morning, I noticed
> some odd behavior with respect to POSIX threads in a test program I had
> written (originally to test epoll.)
> 
> The behavior is as follows:
> 
> 1.  main() creates a new thread of execution with pthread_create
> 2.  thread_func() immediately calls pthread_detach(), which is supposed to
> ensure that thread resources are cleaned up when the thread terminates.
> 3.  The spawned thread sleeps and then prints a message "got here"
> 4.  The main thread calls pthread_join().  According to the POSIX
> documentation, this should suspend execution until the spawned thread has
> terminated.
> 
> What I'm seeing is that the main thread terminates immediately.  If I
> comment out the call to pthread_detach(), the program runs normally.  If I
> boot with my 2.6.23 kernel, the program runs normally.
> 
> I'm not sure if this is some oversight on my part but similar programs
> operate correctly on my Debian etch installation with 2.6.18 as well as
> with the newer 23 kernel, but not with the patch.
> 
> If you have any questions, don't hesitate to email me; my address is
> provided below.
> 
> See Attached (bug.c)
> 
> Tom R. Dial
> tom@...aga.com

Hello Tom

Your program is buggy and depends on scheduling policies.

To make it fails on 2.6.23 (or say 2.6.9) too, just add a sleep(1) before pthread_join() call

Once a thread calls pthread_detach(pthread_t this_id), it's not possible anymore to
pthread_join(this_id, &ret) or effect is undefined.

(quoting POSIX : "The results of multiple simultaneous calls to pthread_join() specifying the same target thread are undefined" )

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