[BUG] kvm: dereference srcu-protected pointer without srcu_read_lock() held

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Applied the patch I just sent and let CONFIG_PROVE_RCU=y,
we can got the following dmesg. And we found that it is
because some codes in KVM dereferences srcu-protected pointer without
srcu_read_lock() held or update-side lock held.

It is not hard to fix, the problem is that:
Where is the most proper place to put a srcu_read_lock()?

I can not determine the answer, so I report this bug
instead of fixing it.

Thanks.
Lai.

Reported-by: Lai Jiangshan <laijs@cn.fujitsu.com>


===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
arch/x86/kvm/x86.h:72 invoked rcu_dereference_check() without protection!

other info that might help us debug this:


rcu_scheduler_active = 1, debug_locks = 0
1 lock held by qemu-system-x86/3027:
 #0:  (&vcpu->mutex){+.+.+.}, at: [<ffffffffa011b6ff>] vcpu_load+0x1a/0x66 [kvm]

stack backtrace:
Pid: 3027, comm: qemu-system-x86 Not tainted 2.6.34-rc4-tip-01028-g939eab1-dirty #28
Call Trace:
 [<ffffffff8106c59e>] lockdep_rcu_dereference+0xaa/0xb3
 [<ffffffffa0123654>] unalias_gfn_instantiation+0x56/0xaf [kvm]
 [<ffffffffa01198de>] gfn_to_hva+0x14/0x4c [kvm]
 [<ffffffffa01199ca>] kvm_write_guest_page+0x2a/0x7f [kvm]
 [<ffffffffa0119afb>] kvm_write_guest+0x41/0x83 [kvm]
 [<ffffffffa01243b5>] kvm_write_guest_virt+0x78/0xa1 [kvm]
 [<ffffffffa012479a>] pio_copy_data+0x46/0x75 [kvm]
 [<ffffffff81385a0f>] ? sub_preempt_count+0x9/0x83
 [<ffffffffa012485a>] complete_pio+0x91/0x1b9 [kvm]
 [<ffffffffa0128728>] kvm_arch_vcpu_ioctl_run+0x93/0xd2b [kvm]
 [<ffffffffa0128f7a>] ? kvm_arch_vcpu_ioctl_run+0x8e5/0xd2b [kvm]
 [<ffffffff8106e590>] ? __lock_acquire+0x7b4/0x16d5
 [<ffffffffa0117ecd>] kvm_vcpu_ioctl+0x103/0x97b [kvm]
 [<ffffffffa011b15b>] ? kvm_vm_ioctl+0x364/0x38d [kvm]
 [<ffffffff810e6b5e>] ? fget_light+0xf1/0x241
 [<ffffffff810f3587>] vfs_ioctl+0x32/0xa6
 [<ffffffff810f3b19>] do_vfs_ioctl+0x495/0x4db
 [<ffffffff810e6c9e>] ? fget_light+0x231/0x241
 [<ffffffff810e6b5e>] ? fget_light+0xf1/0x241
 [<ffffffff810f3ba6>] sys_ioctl+0x47/0x6a
 [<ffffffff810021db>] system_call_fastpath+0x16/0x1b