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Date: Fri, 10 Sep 2004 09:39:27 -0700
From: "H. J. Lu" <hjl@lucon.org>
To: Andrew Morton <akpm@osdl.org>
Cc: linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org
Subject: Re: Hanging process on SMP machines?
Message-ID: <20040910163927.GA8124@lucon.org>
References: <20040909201321.GA21492@lucon.org> <20040910004549.2df18073.akpm@osdl.org>
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On Fri, Sep 10, 2004 at 12:45:49AM -0700, Andrew Morton wrote:
> "H. J. Lu" <hjl@lucon.org> wrote:
> >
> > I notice that a process may hang on SMP machines at random:
> > 
> >  http://bugzilla.kernel.org/show_bug.cgi?id=3332
> > 
> >  I can reliably trigger it within 15 minutes under SMP kernel on P4 HT
> >  and 4-way ia64 machines. Has anyone else seen it?
> 
> It's easy to reproduce on 2-way x86, however it doesn't look like a kernel
> bug.
> 
> 
> akpm      2503  0.0  0.2  3892  640 pts/0    S    00:34   0:00  |           \_ make
> akpm      2504  0.0  0.0  1368  192 pts/0    S    00:34   0:00  |               \_ time expect test.exp
> akpm      2505  0.0  0.5  4776 1276 pts/0    S    00:34   0:00  |                   \_ expect test.exp
> akpm      4726  0.0  0.0     0    0 ?        Z    00:35   0:00  |                       \_ [true] <defunct>
> 
> process 4726 is sleeping at the end of do_exit():
> 
> 
> 	schedule();		<<- here
> 	BUG();
> 	/* Avoid "noreturn function does return".  */
> 	for (;;) ;
> }
> 
> So it has completely exitted and is waiting for someone to reap its exit
> code and stack slot via wait4().  So what is its parent up to?
> 
> 
> (gdb) thread 69
> [Switching to thread 69 (Thread 2505)]#0  0xc01653e9 in do_select (n=5, fds=0xcc8b5fa4, 
>     timeout=0xcc8b5fa0) at fs/select.c:257
> 257                     __timeout = schedule_timeout(__timeout);
> (gdb) bt
> #0  0xc01653e9 in do_select (n=5, fds=0xcc8b5fa4, timeout=0xcc8b5fa0) at fs/select.c:257
> #1  0xc016579c in sys_select (n=5, inp=0x804b444, outp=0x804b4c4, exp=0x804b544, tvp=0xbfffe1d0)
>     at fs/select.c:354
> #2  0xc0105e39 in sysenter_past_esp () at arch/i386/kernel/semaphore.c:177
> 
> The parent is sleeping in select() rather than wait()ing for children.
> 

I don't think SIGCHLD is blocked. Shouldn't SIGCHLD interrupt select?
UP kernel doesn't have this problem.


H.J.
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