From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>, Marc Zyngier <maz@kernel.org>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary
Date: Mon, 19 Apr 2021 15:09:01 +0000 [thread overview]
Message-ID: <YH2dDRBXJcbUcbLi@google.com> (raw)
In-Reply-To: <2a7670e4-94c0-9f35-74de-a7d5b1504ced@redhat.com>
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);
>
next prev parent reply other threads:[~2021-04-19 15:09 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-02 0:56 [PATCH v2 00/10] KVM: Consolidate and optimize MMU notifiers Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 01/10] KVM: Assert that notifier count is elevated in .change_pte() Sean Christopherson
2021-04-02 11:08 ` Paolo Bonzini
2021-04-02 0:56 ` [PATCH v2 02/10] KVM: Move x86's MMU notifier memslot walkers to generic code Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 03/10] KVM: arm64: Convert to the gfn-based MMU notifier callbacks Sean Christopherson
2021-04-12 10:12 ` Marc Zyngier
2021-04-02 0:56 ` [PATCH v2 04/10] KVM: MIPS/MMU: " Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 05/10] KVM: PPC: " Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 06/10] KVM: Kill off the old hva-based " Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 07/10] KVM: Move MMU notifier's mmu_lock acquisition into common helper Sean Christopherson
2021-04-02 9:35 ` Paolo Bonzini
2021-04-02 14:59 ` Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 08/10] KVM: Take mmu_lock when handling MMU notifier iff the hva hits a memslot Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary Sean Christopherson
2021-04-02 9:34 ` Paolo Bonzini
2021-04-02 14:59 ` Sean Christopherson
2021-04-19 8:49 ` Wanpeng Li
2021-04-19 13:50 ` Paolo Bonzini
2021-04-19 15:09 ` Sean Christopherson [this message]
2021-04-19 22:09 ` Paolo Bonzini
2021-04-20 1:17 ` Sean Christopherson
2021-04-02 0:56 ` [PATCH v2 10/10] KVM: x86/mmu: Allow yielding during MMU notifier unmap/zap, if possible Sean Christopherson
2021-04-02 12:17 ` [PATCH v2 00/10] KVM: Consolidate and optimize MMU notifiers Paolo Bonzini
2021-04-12 10:27 ` Marc Zyngier
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