virtio balloon: do not call blocking ops when !TASK_RUNNING

 Hi all,

with the recent kernel 3.19, I get a kernel warning when I start my
KVM guest on s390 with virtio balloon enabled:

[    0.839687] do not call blocking ops when !TASK_RUNNING; state=1 set at
               [<0000000000174a1e>] prepare_to_wait_event+0x7e/0x108
[    0.839694] ------------[ cut here ]------------
[    0.839697] WARNING: at kernel/sched/core.c:7326
[    0.839698] Modules linked in:
[    0.839702] CPU: 0 PID: 46 Comm: vballoon Not tainted 3.19.0 #233
[    0.839705] task: 00000000021d0000 ti: 00000000021d8000 task.ti: 00000000021d8000
[    0.839707] Krnl PSW : 0704c00180000000 000000000015bf8e (__might_sleep+0x8e/0x98)
[    0.839713]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 EA:3
Krnl GPRS: 000000000000000d 00000000021d0000 0000000000000071 0000000000000001
[    0.839718]            0000000000675ace 0000000001998c50 cccccccccccccccc cccccccccccccccc
[    0.839720]            0000000000982134 000000000058f824 0000000000a008a8 0000000000000000
[    0.839722]            00000000000004d9 00000000007ea992 000000000015bf8a 00000000021dbc28
[    0.839731] Krnl Code: 000000000015bf7e: c0200033e838	larl	%r2,7d8fee
           000000000015bf84: c0e50028cd62	brasl	%r14,675a48
          #000000000015bf8a: a7f40001		brc	15,15bf8c
          >000000000015bf8e: 9201a000		mvi	0(%r10),1
           000000000015bf92: a7f4ffe2		brc	15,15bf56
           000000000015bf96: 0707		bcr	0,%r7
           000000000015bf98: ebdff0800024	stmg	%r13,%r15,128(%r15)
           000000000015bf9e: a7f13fe0		tmll	%r15,16352
[    0.839749] Call Trace:
[    0.839751] ([<000000000015bf8a>] __might_sleep+0x8a/0x98)
[    0.839756]  [<000000000028a562>] __kmalloc+0x272/0x350
[    0.839759]  [<000000000058f824>] virtio_ccw_get_config+0x3c/0x100
[    0.839762]  [<000000000049fcb0>] balloon+0x1b8/0x330
[    0.839765]  [<00000000001529c8>] kthread+0x120/0x138
[    0.839767]  [<0000000000683c22>] kernel_thread_starter+0x6/0xc
[    0.839770]  [<0000000000683c1c>] kernel_thread_starter+0x0/0xc
[    0.839772] no locks held by vballoon/46.
[    0.839773] Last Breaking-Event-Address:
[    0.839776]  [<000000000015bf8a>] __might_sleep+0x8a/0x98
[    0.839778] ---[ end trace d27fcdfa27273d7c ]---

The problem seems to be this code in balloon() in
drivers/virtio/virtio_balloon.c:

	wait_event_interruptible(vb->config_change,
				 (diff = towards_target(vb)) != 0
				 || vb->need_stats_update
				 || kthread_should_stop()
				 || freezing(current));

wait_event_interruptible() sets the state of the current task to
TASK_INTERRUPTIBLE, then checks the condition. The condition contains
towards_target() which reads the virtio config space via virtio_cread().
On s390, this then triggers virtio_ccw_get_config() - and this function
calls some other functions again that might sleep (e.g. kzalloc or
wait_event in ccw_io_helper) ... and this causes the new kernel warning
message with kernel 3.19.

I think it would be quite difficult or at least ugly to rewrite
virtio_ccw_get_config() so that it does not call sleepable functions
anymore. So would it be feasible to rewrite the balloon() function that
it does not call the towards_target() in its wait_event condition
anymore? I am unfortunately not that familiar with the balloon code
semantics, so any help is very appreciated here!

 Thanks,
  Thomas