[PATCH-next] kvm: don't try to take mmu_lock while holding the main raw kvm_lock

[Paul Gortmaker <paul.gortmaker@windriver.com>]

In commit e935b8372cf8 ("KVM: Convert kvm_lock to raw_spinlock"),
the kvm_lock was made a raw lock.  However, the kvm mmu_shrink()
function tries to grab the (non-raw) mmu_lock within the scope of
the raw locked kvm_lock being held.  This leads to the following:

BUG: sleeping function called from invalid context at kernel/rtmutex.c:659
in_atomic(): 1, irqs_disabled(): 0, pid: 55, name: kswapd0
Preemption disabled at:[<ffffffffa0376eac>] mmu_shrink+0x5c/0x1b0 [kvm]

Pid: 55, comm: kswapd0 Not tainted 3.4.34_preempt-rt
Call Trace:
 [<ffffffff8106f2ad>] __might_sleep+0xfd/0x160
 [<ffffffff817d8d64>] rt_spin_lock+0x24/0x50
 [<ffffffffa0376f3c>] mmu_shrink+0xec/0x1b0 [kvm]
 [<ffffffff8111455d>] shrink_slab+0x17d/0x3a0
 [<ffffffff81151f00>] ? mem_cgroup_iter+0x130/0x260
 [<ffffffff8111824a>] balance_pgdat+0x54a/0x730
 [<ffffffff8111fe47>] ? set_pgdat_percpu_threshold+0xa7/0xd0
 [<ffffffff811185bf>] kswapd+0x18f/0x490
 [<ffffffff81070961>] ? get_parent_ip+0x11/0x50
 [<ffffffff81061970>] ? __init_waitqueue_head+0x50/0x50
 [<ffffffff81118430>] ? balance_pgdat+0x730/0x730
 [<ffffffff81060d2b>] kthread+0xdb/0xe0
 [<ffffffff8106e122>] ? finish_task_switch+0x52/0x100
 [<ffffffff817e1e94>] kernel_thread_helper+0x4/0x10
 [<ffffffff81060c50>] ? __init_kthread_worker+0x

Since we only use the lock for protecting the vm_list, once we've
found the instance we want, we can shuffle it to the end of the
list and then drop the kvm_lock before taking the mmu_lock.  We
can do this because after the mmu operations are completed, we
break -- i.e. we don't continue list processing, so it doesn't
matter if the list changed around us.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---

[Note1: do double check that this solution makes sense for the
 mainline kernel; consider this an RFC patch that does want a
 review from people in the know.]

[Note2: you'll need to be running a preempt-rt kernel to actually
 see this.  Also note that the above patch is against linux-next.
 Alternate solutions welcome ; this seemed to me the obvious fix.]

 arch/x86/kvm/mmu.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 748e0d8..db93a70 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4322,6 +4322,7 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 {
 	struct kvm *kvm;
 	int nr_to_scan = sc->nr_to_scan;
+	int found = 0;
 	unsigned long freed = 0;
 
 	raw_spin_lock(&kvm_lock);
@@ -4349,6 +4350,12 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 			continue;
 
 		idx = srcu_read_lock(&kvm->srcu);
+
+		list_move_tail(&kvm->vm_list, &vm_list);
+		found = 1;
+		/* We can't be holding a raw lock and take non-raw mmu_lock */
+		raw_spin_unlock(&kvm_lock);
+
 		spin_lock(&kvm->mmu_lock);
 
 		if (kvm_has_zapped_obsolete_pages(kvm)) {
@@ -4370,11 +4377,12 @@ unlock:
 		 * per-vm shrinkers cry out
 		 * sadness comes quickly
 		 */
-		list_move_tail(&kvm->vm_list, &vm_list);
 		break;
 	}
 
-	raw_spin_unlock(&kvm_lock);
+	if (!found)
+		raw_spin_unlock(&kvm_lock);
+
 	return freed;
 
 }
-- 
1.8.1.2