Re: [bug report] KVM: x86/mmu: Use an rwlock for the x86 MMU

[David Matlack <dmatlack@google.com>]

On Mon, Jul 26, 2021 at 10:52:38AM +0300, Dan Carpenter wrote:
> [ This is not the correct patch to blame, but there is something going
>   on here which I don't understand so this email is more about me
>   learning rather than reporting bugs. - dan ]
> 
> Hello Ben Gardon,
> 
> The patch 531810caa9f4: "KVM: x86/mmu: Use an rwlock for the x86 MMU"
> from Feb 2, 2021, leads to the following static checker warning:
> 
> 	arch/x86/kvm/mmu/mmu.c:5769 kvm_mmu_zap_all()
> 	warn: sleeping in atomic context
> 
> arch/x86/kvm/mmu/mmu.c
>     5756 void kvm_mmu_zap_all(struct kvm *kvm)
>     5757 {
>     5758 	struct kvm_mmu_page *sp, *node;
>     5759 	LIST_HEAD(invalid_list);
>     5760 	int ign;
>     5761 
>     5762 	write_lock(&kvm->mmu_lock);
>                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
> This line bumps the preempt count.
> 
>     5763 restart:
>     5764 	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
>     5765 		if (WARN_ON(sp->role.invalid))
>     5766 			continue;
>     5767 		if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))
>     5768 			goto restart;
> --> 5769 		if (cond_resched_rwlock_write(&kvm->mmu_lock))
>                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This line triggers a sleeping in atomic warning.  What's going on here
> that I'm not understanding?

cond_resched_rwlock_write drops the provided lock (kvm->mmu_lock in this
case) before scheduling and then re-acquires it afterwards. So this
warning looks like a false positive.

> 
>     5770 			goto restart;
>     5771 	}
>     5772 
>     5773 	kvm_mmu_commit_zap_page(kvm, &invalid_list);
>     5774 
>     5775 	if (is_tdp_mmu_enabled(kvm))
>     5776 		kvm_tdp_mmu_zap_all(kvm);
>     5777 
>     5778 	write_unlock(&kvm->mmu_lock);
>     5779 }
> 
> regards,
> dan carpenter