BUG: sleeping function called from invalid context

[Dario Faggioli <raistlin@linux.it>]

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Hello everyone,

Since I really plan to spend some time here, let me introduce myself
first. My name is Dario Faggioli and I just joined the Citrix Platform
Team in Cambridge (although I'll be working from Italy). I've some
experience in Linux kernel (mainly scheduling) and not that much
experience in Xen or virtualization in general, but I really want to
learn and be able to contribute ASAP!

In fact, while "doing my homework", I stumbled against the following
BUG(). I'm able to reproduce it with xen-unstable and by just by
enabling spinlock and mutex debug checks in linus' Linux
(65112dccf8a113737684366349d7f9ec373ddc47) _iff_ using 'tap:qcow2' disk
image, while no such thing happens with 'phy:'.

[  996.282544] BUG: sleeping function called from invalid context at /local/scratch/dariof/linux/kernel/mutex.c:271
[  996.282570] in_atomic(): 1, irqs_disabled(): 0, pid: 3256, name: qemu-dm
[  996.282581] 1 lock held by qemu-dm/3256:
[  996.282589]  #0:  (&(&priv->lock)->rlock){......}, at: [<ffffffff813223da>] gntdev_ioctl+0x2bd/0x4d5
[  996.282628] Pid: 3256, comm: qemu-dm Tainted: G        W   3.1.0-rc8+ #5
[  996.282638] Call Trace:
[  996.282687]  [<ffffffff81054594>] __might_sleep+0x131/0x135
[  996.282704]  [<ffffffff816bd64f>] mutex_lock_nested+0x25/0x45
[  996.282721]  [<ffffffff8131c7c8>] free_xenballooned_pages+0x20/0xb1
[  996.282735]  [<ffffffff8132194d>] gntdev_put_map+0xa8/0xdb
[  996.282749]  [<ffffffff816be546>] ? _raw_spin_lock+0x71/0x7a
[  996.282763]  [<ffffffff813223da>] ? gntdev_ioctl+0x2bd/0x4d5
[  996.282776]  [<ffffffff8132243c>] gntdev_ioctl+0x31f/0x4d5
[  996.282790]  [<ffffffff81007d62>] ? check_events+0x12/0x20
[  996.282804]  [<ffffffff811433bc>] do_vfs_ioctl+0x488/0x4d7
[  996.282818]  [<ffffffff81007d4f>] ? xen_restore_fl_direct_reloc+0x4/0x4
[  996.282832]  [<ffffffff8109168b>] ? lock_release+0x21c/0x229
[  996.282847]  [<ffffffff81135cdd>] ? rcu_read_unlock+0x21/0x32
[  996.282860]  [<ffffffff81143452>] sys_ioctl+0x47/0x6a
[  996.282873]  [<ffffffff816bfd82>] system_call_fastpath+0x16/0x1b

This seems to be due to free_xenballooned_pages(), called by
gntdev_put_map(), taking balloon_mutex, with the latter that can be
called within a spin_lock() (e.g., in gntdev_release()).

I'm not enough confident with the code do attempt fixing it, but I
thought it was worth to at least point it out!

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)
PhD Candidate, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

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