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Date:	Thu, 4 Nov 2010 13:47:18 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [regression, 2.6.37-rc1] 'ip link tap0 up' stuck in do_exit()

On Thu, Nov 04, 2010 at 11:21:40AM +1100, Dave Chinner wrote:
>On Wed, Nov 03, 2010 at 10:29:36PM +1100, Dave Chinner wrote:
>> On Wed, Nov 03, 2010 at 09:34:48PM +1100, Dave Chinner wrote:
>> > On Wed, Nov 03, 2010 at 08:13:22AM +0100, Eric Dumazet wrote:
>> > > Le mercredi 03 novembre 2010 à 17:26 +1100, Dave Chinner a écrit :
>> > > > Folks,
>> > > > 
>> > > > Starting up KVM on a current mainline kernel using the tap
>> > > > device for the networking is resulting in the ip process tryin gto
>> > > > up the tap interface hanging. KVM is started with this networking
>> > > > config:
>> > > > 
>> > > > ....
>> > > >         -net nic,vlan=0,macaddr=00:e4:b6:63:63:6d,model=virtio \
>> > > >         -net tap,vlan=0,script=/vm-images/qemu-ifup,downscript=no \
>> > > > ....
>> > > > 
>> > > > And the script is effectively:
>> > > > 
>> > > > switch=br0
>> > > > if [ -n "$1" ];then
>> > > >         /usr/bin/sudo /sbin/ip link set $1 up
>> > > >         sleep 0.5s
>> > > >         /usr/bin/sudo /usr/sbin/brctl addif $switch $1
>> > > > 	exit 0
>> > > > fi
>> > > > exit 1
>> > > > 
>> > > > This is resulting in the command 'ip link set tap0 up' hanging as a zombie:
>> > > > 
>> > > > root      3005     1  0 16:53 pts/3    00:00:00 /bin/sh /vm-images/qemu-ifup tap0
>> > > > root      3011  3005  0 16:53 pts/3    00:00:00 /usr/bin/sudo /sbin/ip link set tap0 up
>> > > > root      3012  3011  0 16:53 pts/3    00:00:00 [ip] <defunct>
>> > > > 
>> > > > In do_exit() with this trace:
>> > > > 
>> > > > [ 1630.782255] ip            x ffff88063fcb3600     0  3012   3011 0x00000000
>> > > > [ 1630.789121]  ffff880631328000 0000000000000046 0000000000000000 ffff880633104380
>> > > > [ 1630.796524]  0000000000013600 ffff88062f031fd8 0000000000013600 0000000000013600
>> > > > [ 1630.803925]  ffff8806313282d8 ffff8806313282e0 ffff880631328000 0000000000013600
>> > > > [ 1630.811324] Call Trace:
>> > > > [ 1630.813760]  [<ffffffff8104a90d>] ? do_exit+0x716/0x724
>> > > > [ 1630.818964]  [<ffffffff8104a995>] ? do_group_exit+0x7a/0xa4
>> > > > [ 1630.824512]  [<ffffffff8104a9d1>] ? sys_exit_group+0x12/0x16
>> > > > [ 1630.830149]  [<ffffffff81009a82>] ? system_call_fastpath+0x16/0x1b
>> > > > 
>> > > > The address comes down to the schedule() call:
>> > > > 
>> > > > (gdb) l *(do_exit+0x716)
>> > > > 0xffffffff8104a90d is in do_exit (kernel/exit.c:1034).
>> > > > 1029            preempt_disable();
>> > > > 1030            exit_rcu();
>> > > > 1031            /* causes final put_task_struct in finish_task_switch(). */
>> > > > 1032            tsk->state = TASK_DEAD;
>> > > > 1033            schedule();
>> > > > 1034            BUG();
>> > > > 1035            /* Avoid "noreturn function does return".  */
>> > > > 1036            for (;;)
>> > > > 1037                    cpu_relax();    /* For when BUG is null */
>> > > > 1038    }
>> > > > 
>> > > > Needless to say, KVM is not starting up. This works just fine on
>> > > > 2.6.35.1 and so is a regression. I can't do a lot of testing on this as
>> > > > the host is the machine that hosts all my build and test environments....
>> > > > 
>> > > > Cheers,
>> > > > 
>> > > > Dave.
>> > > 
>> > > Could it be the same problem than 
>> > > 
>> > > http://kerneltrap.com/mailarchive/linux-netdev/2010/10/23/6288128
>> > > 
>> > > Try to revert bee31369ce16fc3898ec9a54161248c9eddb06bc ?
>> > 
>> > It's working fine on 2.6.36 right now, so it's something that came in
>> > with the .37 merge cycle...
>> 
>> Actually, the machine isn't running a 2.6.36 kernel (it had booted
>> to the working .35 kernel and I didn't notice). So i've just tested
>> a 2.6.36 kernel, and the problem _is present_ in 2.6.36. I've
>> reverted the above commit but that does not fix the problem.
>
>Ok, so further investigation has shown I can reproduce this on
>2.6.32 and 2.6.35. It's not a new bug, nor do I think that it is
>a networking bug as it is not specific to the ip command.
>
>The trigger for the problem is actually an upgrade of the sudo
>package in debian unstable which changed the behaviour of sudo (has
>some per-login/pty restriction on it now). Basically, the startup
>script I'm running does:
>
>sudo kvm .....
>
>which then executes the qemu-ifup bash script which does:
>
>	sudo ip ....
>	sudo brctl ...
>
>because at one point KVM did not create the tap device automatically
>and so kvm could be run as a user with only the ifup script
>requiring privileges to create the tap device and mark it up. When
>KVM started creating the tap device, I added the sudo to the KVM
>script, an everything worked again.
>
>Now if I take the 'sudo' out of the ifup script, the hang goes away.
>I first removed it from the ip command, and then the brctl command
>hung in the same way the ip command was hanging. Hence my thoughts
>that it is not directly related to networking utilities.
>Unfortunately, it is not trivial to reproduce as I could only
>trigger it through this kvm method, not on the command line. e.g:
>
>$ sudo bash -c "sudo ip link set tap1 up"
>
>does not hang.
>
>This sudo package upgrade coincided with kernel upgrades, and so
>that lead to my confusion about where it occurred and what triggered
>it.  Still, it appears to be a bug that has been around for some
>time.....
>

Interesting, the scheduler failed to put the dead task out of
run queue, so to me this is likely to be a scheduler bug.
I have no idea how sudo can change the behaviour here.

Another guess is we need a smp_wmb() before schedule() above.

We need to Cc Oleg and Ingo.
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