On Mon, Apr 19, 2021, Paolo Bonzini wrote:
> 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.
Ugh, the count approach is nearly identical to Ben's original code. Using a
rwsem seemed so clever :-/
> IIUC this could be extended to non-sleeping invalidations too, but I
> am not really sure about that.
Yes, that should be fine.
> 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
If using spinlock_t is problematic, wouldn't mmu_lock already be an issue? Or
am I misunderstanding your concern?
> - 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.
The one concern I had with an unfair mechanism of this nature is that, in theory,
the memslot update could be blocked indefinitely.
> 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.
I think delaying it makes sense.
> @@ -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.
How on earth did you decipher that from the splat? I stared at it for a good
five minutes and was completely befuddled.
> + */
> + 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);
>
