[regression, 2.6.37-rc1] 'ip link tap0 up' stuck in do_exit()

[Dave Chinner <david@fromorbit.com>]

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.
-- 
Dave Chinner
david@fromorbit.com