Messages in this thread Patch in this message | | | From | Paolo Bonzini <> | Subject | Re: [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary | Date | Mon, 19 Apr 2021 15:50:25 +0200 |
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On 19/04/21 10:49, Wanpeng Li wrote: > I saw this splatting: > > ====================================================== > WARNING: possible circular locking dependency detected > 5.12.0-rc3+ #6 Tainted: G OE > ------------------------------------------------------ > qemu-system-x86/3069 is trying to acquire lock: > ffffffff9c775ca0 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}, > at: __mmu_notifier_invalidate_range_end+0x5/0x190 > > but task is already holding lock: > ffffaff7410a9160 (&kvm->mmu_notifier_slots_lock){.+.+}-{3:3}, at: > kvm_mmu_notifier_invalidate_range_start+0x36d/0x4f0 [kvm]
I guess it is possible to open-code the wait using a readers count and a spinlock (see patch after signature). This allows including the rcu_assign_pointer in the same critical section that checks the number of readers. Also on the plus side, the init_rwsem() is replaced by slightly nicer code.
IIUC this could be extended to non-sleeping invalidations too, but I am not really sure about that.
There are some issues with the patch though:
- I am not sure if this should be a raw spin lock to avoid the same issue on PREEMPT_RT kernel. That said the critical section is so tiny that using a raw spin lock may make sense anyway
- this loses the rwsem fairness. On the other hand, mm/mmu_notifier.c's own interval-tree-based filter is also using a similar mechanism that is likewise not fair, so it should be okay.
Any opinions? For now I placed the change below in kvm/queue, but I'm leaning towards delaying this optimization to the next merge window.
Paolo
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst index 8f5d5bcf5689..e628f48dfdda 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -16,12 +16,11 @@ The acquisition orders for mutexes are as follows: - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring them together is quite rare. -- The kvm->mmu_notifier_slots_lock rwsem ensures that pairs of +- kvm->mn_active_invalidate_count ensures that pairs of invalidate_range_start() and invalidate_range_end() callbacks - use the same memslots array. kvm->slots_lock is taken outside the - write-side critical section of kvm->mmu_notifier_slots_lock, so - MMU notifiers must not take kvm->slots_lock. No other write-side - critical sections should be added. + use the same memslots array. kvm->slots_lock is taken on the + waiting side in install_new_memslots, so MMU notifiers must not + take kvm->slots_lock. On x86: diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 76b340dd6981..44a4a0c5148a 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -472,11 +472,15 @@ struct kvm { #endif /* KVM_HAVE_MMU_RWLOCK */ struct mutex slots_lock; - struct rw_semaphore mmu_notifier_slots_lock; struct mm_struct *mm; /* userspace tied to this vm */ struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; struct kvm_vcpu *vcpus[KVM_MAX_VCPUS]; + /* Used to wait for completion of MMU notifiers. */ + spinlock_t mn_invalidate_lock; + unsigned long mn_active_invalidate_count; + struct rcuwait mn_memslots_update_rcuwait; + /* * created_vcpus is protected by kvm->lock, and is incremented * at the beginning of KVM_CREATE_VCPU. online_vcpus is only @@ -662,7 +666,7 @@ static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id) as_id = array_index_nospec(as_id, KVM_ADDRESS_SPACE_NUM); return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu, lockdep_is_held(&kvm->slots_lock) || - lockdep_is_held(&kvm->mmu_notifier_slots_lock) || + READ_ONCE(kvm->mn_active_invalidate_count) || !refcount_read(&kvm->users_count)); } diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index ff9e95eb6960..cdaa1841e725 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -624,7 +624,7 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn, * otherwise, mmu_notifier_count is incremented unconditionally. */ if (!kvm->mmu_notifier_count) { - lockdep_assert_held(&kvm->mmu_notifier_slots_lock); + WARN_ON(!READ_ONCE(kvm->mn_active_invalidate_count)); return; } @@ -689,10 +689,13 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn, * The complexity required to handle conditional locking for this case * is not worth the marginal benefits, the VM is likely doomed anyways. * - * Pairs with the up_read in range_end(). + * Pairs with the decrement in range_end(). */ - if (blockable) - down_read(&kvm->mmu_notifier_slots_lock); + if (blockable) { + spin_lock(&kvm->mn_invalidate_lock); + kvm->mn_active_invalidate_count++; + spin_unlock(&kvm->mn_invalidate_lock); + } __kvm_handle_hva_range(kvm, &hva_range); @@ -735,9 +738,20 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, __kvm_handle_hva_range(kvm, &hva_range); - /* Pairs with the down_read in range_start(). */ - if (blockable) - up_read(&kvm->mmu_notifier_slots_lock); + /* Pairs with the increment in range_start(). */ + if (blockable) { + bool wake; + spin_lock(&kvm->mn_invalidate_lock); + wake = (--kvm->mn_active_invalidate_count == 0); + spin_unlock(&kvm->mn_invalidate_lock); + + /* + * There can only be one waiter, since the wait happens under + * slots_lock. + */ + if (wake) + rcuwait_wake_up(&kvm->mn_memslots_update_rcuwait); + } BUG_ON(kvm->mmu_notifier_count < 0); } @@ -951,7 +965,9 @@ static struct kvm *kvm_create_vm(unsigned long type) mutex_init(&kvm->lock); mutex_init(&kvm->irq_lock); mutex_init(&kvm->slots_lock); - init_rwsem(&kvm->mmu_notifier_slots_lock); + spin_lock_init(&kvm->mn_invalidate_lock); + rcuwait_init(&kvm->mn_memslots_update_rcuwait); + INIT_LIST_HEAD(&kvm->devices); BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX); @@ -1073,15 +1089,17 @@ static void kvm_destroy_vm(struct kvm *kvm) #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm); /* - * Reset the lock used to prevent memslot updates between MMU notifier - * invalidate_range_start() and invalidate_range_end(). At this point, - * no more MMU notifiers will run and pending calls to ...start() have - * completed. But, the lock could still be held if KVM's notifier was - * removed between ...start() and ...end(). No threads can be waiting - * on the lock as the last reference on KVM has been dropped. If the - * lock is still held, freeing memslots will deadlock. + * At this point, pending calls to invalidate_range_start() + * have completed but no more MMU notifiers will run, so + * mn_active_invalidate_count may remain unbalanced. + * No threads can be waiting in install_new_memslots as the + * last reference on KVM has been dropped, but freeing + * memslots will deadlock without manual intervention. */ - init_rwsem(&kvm->mmu_notifier_slots_lock); + spin_lock(&kvm->mn_invalidate_lock); + kvm->mn_active_invalidate_count = 0; + WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait)); + spin_unlock(&kvm->mn_invalidate_lock); #else kvm_arch_flush_shadow_all(kvm); #endif @@ -1333,9 +1351,22 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, WARN_ON(gen & KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS); slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS; - down_write(&kvm->mmu_notifier_slots_lock); + /* + * This cannot be an rwsem because the MMU notifier must not run + * inside the critical section. A sleeping rwsem cannot exclude + * that. + */ + spin_lock(&kvm->mn_invalidate_lock); + prepare_to_rcuwait(&kvm->mn_memslots_update_rcuwait); + while (kvm->mn_active_invalidate_count) { + set_current_state(TASK_UNINTERRUPTIBLE); + spin_unlock(&kvm->mn_invalidate_lock); + schedule(); + spin_lock(&kvm->mn_invalidate_lock); + } + finish_rcuwait(&kvm->mn_memslots_update_rcuwait); rcu_assign_pointer(kvm->memslots[as_id], slots); - up_write(&kvm->mmu_notifier_slots_lock); + spin_unlock(&kvm->mn_invalidate_lock); synchronize_srcu_expedited(&kvm->srcu);
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