Re: Hanging process on SMP machines?

[Andrew Morton <akpm@osdl.org>]

"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?id332
> 
>  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.


(gdb) f 0
#0  0xc01653e9 in do_select (n=5, fds=0xcc8b5fa4, timeout=0xcc8b5fa0) at fs/select.c:257
257                     __timeout = schedule_timeout(__timeout);
(gdb) p __timeout
$1 = 3000000

For 3,000 seconds.


I changed your 3000 to 30 and lo, the script hangs for 30 seconds every now
and then, and then resumes.