From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Date: Wed, 31 Mar 2021 20:15:59 +0000 Subject: Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary Message-Id: List-Id: References: <20210326021957.1424875-1-seanjc@google.com> <20210326021957.1424875-17-seanjc@google.com> <6e7dc7d0-f5dc-85d9-1c50-d23b761b5ff3@redhat.com> In-Reply-To: <6e7dc7d0-f5dc-85d9-1c50-d23b761b5ff3@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Paolo Bonzini Cc: Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , James Morse , Julien Thierry , Suzuki K Poulose , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, Ben Gardon On Wed, Mar 31, 2021, Paolo Bonzini wrote: > On 26/03/21 03:19, Sean Christopherson wrote: > > + /* > > + * Reset the lock used to prevent memslot updates between MMU notifier > > + * range_start and range_end. At this point no more MMU notifiers will > > + * run, but the lock could still be held if KVM's notifier was removed > > + * between range_start and range_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. > > + */ > > + init_rwsem(&kvm->mmu_notifier_slots_lock); > > I was going to say that this is nasty, then I noticed that > mmu_notifier_unregister uses SRCU to ensure completion of concurrent calls > to the MMU notifier. So I guess it's fine, but it's better to point it out: > > /* > * At this point no more MMU notifiers will run and pending > * calls to range_start have completed, but the lock would > * still be held and never released if the MMU notifier was > * removed between range_start and range_end. Since the last > * reference to the struct kvm has been dropped, no threads can > * be waiting on the lock, but we might still end up taking it > * when freeing memslots in kvm_arch_destroy_vm. Reset the lock > * to avoid deadlocks. > */ An alternative would be to not take the lock in install_new_memslots() if kvm->users_count = 0. It'd be weirder to document, and the conditional locking would still be quite ugly. Not sure if that's better than blasting a lock during destruction?