On Tue, Apr 25, 2023, David Matlack wrote: > On Mon, Apr 24, 2023 at 05:36:37PM -0700, Sean Christopherson wrote: > > On Mon, Apr 24, 2023, David Matlack wrote: > > > It'd be nice to keep around the lockdep assertion though for the other (and > > > future) callers. The cleanest options I can think of are: > > > > > > 1. Pass in a bool "vm_teardown" kvm_tdp_mmu_invalidate_all_roots() and > > > use that to gate the lockdep assertion. > > > 2. Take the mmu_lock for read in kvm_mmu_uninit_tdp_mmu() and pass > > > down bool shared to kvm_tdp_mmu_invalidate_all_roots(). > > > > > > Both would satisfy your concern of not blocking teardown on the async > > > worker and my concern of keeping the lockdep check. I think I prefer > > > (1) since, as you point out, taking the mmu_lock at all is > > > unnecessary. > > > > Hmm, another option: > > > > 3. Refactor the code so that kvm_arch_init_vm() doesn't call > > kvm_tdp_mmu_invalidate_all_roots() when VM creation fails, and then lockdep > > can ignore on users_count==0 without hitting the false positive. > > > > I like (2) the least. Not sure I prefer (1) versus (3). I dislike passing bools > > just to ignore lockdep, but reworking code for a "never hit in practice" edge case > > is arguably worse :-/ > > Agree (2) is the worst option. (3) seems potentially brittle (likely to > trigger a false-positive lockdep warning if the code ever gets > refactored back). > > How about throwing some underscores at the problem? LOL, now we're speaking my language. I think I have a better option though. The false positives on users_count can be suppressed by gating the assert on kvm->created_vcpus. If KVM_CREATE_VM fails then it's impossible for the VM to have created vCPUs. I like this option in particular because it captures why it's safe for the KVM_CREATE_VM error path to run without mmu_lock (no vCPUs == no roots). I'll manually test this against the error path tomorrow: if (IS_ENABLED(CONFIG_PROVE_LOCKING) && refcount_read(&kvm->users_count) && kvm->created_vcpus) lockdep_assert_held_write(&kvm->mmu_lock);
