On Wed, Mar 31, 2021, Paolo Bonzini wrote: > On 31/03/21 23:05, Sean Christopherson wrote: > > > Wouldn't it be incorrect to lock a mutex (e.g. inside*another* MMU > > > notifier's invalidate callback) while holding an rwlock_t? That makes sense > > > because anybody that's busy waiting in write_lock potentially cannot be > > > preempted until the other task gets the mutex. This is a potential > > > deadlock. > > > > Yes? I don't think I follow your point though. Nesting a spinlock or rwlock > > inside a rwlock is ok, so long as the locks are always taken in the same order, > > i.e. it's never mmu_lock -> mmu_notifier_slots_lock. > > *Another* MMU notifier could nest a mutex inside KVM's rwlock. > > But... is it correct that the MMU notifier invalidate callbacks are always > called with the mmap_sem taken (sometimes for reading, e.g. > try_to_merge_with_ksm_page->try_to_merge_one_page->write_protect_page)? No :-( File-based invalidations through the rmaps do not take mmap_sem. They get at the VMAs via the address_space's interval tree, which is protected by its own i_mmap_rwsem. E.g. try_to_unmap() -> rmap_walk_file() -> try_to_unmap_one() > We could take it temporarily in install_memslots, since the MMU notifier's mm > is stored in kvm->mm. > > In this case, a pair of kvm_mmu_notifier_lock/unlock functions would be the > best way to abstract it. > > Paolo >
