On Tue, Jun 25, 2013 at 06:34:03PM -0400, Paul Gortmaker wrote:
> In commit e935b8372cf8 ("KVM: Convert kvm_lock to raw_spinlock"),
I am copying Jan, the author of the patch. Commit message says:
"Code under this lock requires non-preemptibility", but which code
exactly is this? Is this still true?
> 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@xxxxxxxxxxxxx>
> ---
>
> [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
--
Gleb.
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