On Thu, Mar 30, 2023, Yan Zhao wrote: > On Wed, Mar 29, 2023 at 01:51:23PM +0200, Paolo Bonzini wrote: > > On 3/29/23 03:53, Yan Zhao wrote: > > > Yes, there's no actual deadlock currently. > > > > > > But without fixing this issue, debug_locks will be set to false along > > > with below messages printed. Then lockdep will be turned off and any > > > other lock detections like lockdep_assert_held()... will not print > > > warning even when it's obviously violated. > > > > Can you use lockdep subclasses, giving 0 to the sched_in path and 1 to the > > sched_out path? > > Yes, thanks for the suggestion! > This can avoid this warning of "possible circular locking dependency". > > I tried it like this: > - in sched_out path: > raw_spin_lock_nested(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu), 1); > > - in irq and sched_in paths: > raw_spin_lock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); > > But I have a concern: > If sched_in path removes vcpu A from wakeup list of its previous pcpu A, > and at the mean time, sched_out path adds vcpu B to the wakeup list of > pcpu A, the sched_in and sched_out paths should race for the same > subclass of lock. > But if sched_in path only holds subclass 0, and sched_out path holds > subclass 1, then lockdep would not warn of "possible circular locking > dependency" if someone made a change as below in sched_in path. > > if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR) { > raw_spin_lock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); > list_del(&vmx->pi_wakeup_list); > + raw_spin_lock(¤t->pi_lock); > + raw_spin_unlock(¤t->pi_lock); > raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); > } > > While with v3 of this patch (sched_in path holds both out_lock and in_lock), > lockdep is still able to warn about this issue. Couldn't we just add a manual assertion? That'd also be a good location for a comment to document all of this, and to clarify that current->pi_lock is a completely different lock that has nothing to do with posted interrupts. It's not foolproof, but any patches that substantially touch this code need a ton of scrutiny as the scheduling interactions are gnarly, i.e. IMO a deadlock bug sneaking in is highly unlikely. diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c index 94c38bea60e7..19325a10e42f 100644 --- a/arch/x86/kvm/vmx/posted_intr.c +++ b/arch/x86/kvm/vmx/posted_intr.c @@ -90,6 +90,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu) */ if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR) { raw_spin_lock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); + lockdep_assert_not_held(¤t->pi_lock); list_del(&vmx->pi_wakeup_list); raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu)); }