[[PATCH 2/2] kvm: enable preemption to register/unregister preempt notifier

[Tiejun Chen <tiejun.chen@intel.com>]

After commit 1cde2930e154 ("sched/preempt: Add static_key() to
preempt_notifiers") is introduced, preempt_notifier_{register, unregister}
always hold a mutex, jump_label_mutex. So in current case this shouldn't
work further under the circumstance of disabled preemption, and its also
safe since we're just handling a per-vcpu stuff with holding vcpu->mutex.
Otherwise, some warning messages are posted like this,

BUG: scheduling while atomic: qemu-system-x86/17177/0x00000002
2 locks held by qemu-system-x86/17177:
 #0:  (&vcpu->mutex){+.+.+.}, at: [<ffffffffa035fb48>] vcpu_load+0x28/0xf0 [kvm]
 #1:  (jump_label_mutex){+.+.+.}, at: [<ffffffff81244b54>] static_key_slow_inc+0xc4/0x140
Modules linked in: x86_pkg_temp_thermal kvm_intel kvm
Preemption disabled at:[<ffffffffa035fd3e>] kvm_vcpu_ioctl+0x7e/0xeb0 [kvm]

CPU: 2 PID: 17177 Comm: qemu-system-x86 Tainted: G        W       4.1.0+ #30
Hardware name: Dell Inc. OptiPlex 9020/0DNKMN, BIOS A05 12/05/2013
 0000000000200206 ffff8801c584bc38 ffffffff81f974ab 0000000000000003
 ffff880211289a80 ffff8801c584bc58 ffffffff81f8fd3e 0000000000000001
 ffff8802161d5d00 ffff8801c584bcb8 ffffffff81fa43dc ffff8801c584bd68
Call Trace:
 [<ffffffff81f974ab>] dump_stack+0x95/0xf2
 [<ffffffff81f8fd3e>] __schedule_bug+0x108/0x126
 [<ffffffff81fa43dc>] __schedule+0x12dc/0x1590
 [<ffffffff81fa4965>] schedule+0x75/0x150
 [<ffffffff81fa7683>] ? mutex_lock_nested+0x393/0x780
 [<ffffffff81fa4db0>] schedule_preempt_disabled+0x30/0x60
 [<ffffffff81fa753a>] mutex_lock_nested+0x24a/0x780
 [<ffffffff81244b54>] ? static_key_slow_inc+0xc4/0x140
 [<ffffffff81244b54>] ? static_key_slow_inc+0xc4/0x140
 [<ffffffffa035fb48>] ? vcpu_load+0x28/0xf0 [kvm]
 [<ffffffff81244b54>] static_key_slow_inc+0xc4/0x140
 [<ffffffff810d36a5>] preempt_notifier_register+0x25/0x70
 [<ffffffffa035fb96>] vcpu_load+0x76/0xf0 [kvm]
 [<ffffffffa035fd3e>] kvm_vcpu_ioctl+0x7e/0xeb0 [kvm]
 [<ffffffff8110bc40>] ? __lock_is_held+0x70/0xa0
 [<ffffffff810dde79>] ? get_parent_ip+0x19/0x90
 [<ffffffff8130e4b4>] do_vfs_ioctl+0x3c4/0x910
 [<ffffffff81320a01>] ? expand_files+0x311/0x360
 [<ffffffff815370af>] ? selinux_file_ioctl+0x6f/0x150
 [<ffffffff8130eaad>] SyS_ioctl+0xad/0xe0
 [<ffffffff81faf857>] entry_SYSCALL_64_fastpath+0x12/0x6f

CC: Gleb Natapov <gleb@kernel.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
---
 virt/kvm/kvm_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 848af90..bde5f66f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -127,8 +127,8 @@ int vcpu_load(struct kvm_vcpu *vcpu)
 
 	if (mutex_lock_killable(&vcpu->mutex))
 		return -EINTR;
-	cpu = get_cpu();
 	preempt_notifier_register(&vcpu->preempt_notifier);
+	cpu = get_cpu();
 	kvm_arch_vcpu_load(vcpu, cpu);
 	put_cpu();
 	return 0;
@@ -138,8 +138,8 @@ void vcpu_put(struct kvm_vcpu *vcpu)
 {
 	preempt_disable();
 	kvm_arch_vcpu_put(vcpu);
-	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	preempt_enable();
+	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	mutex_unlock(&vcpu->mutex);
 }
 
-- 
1.9.1