[PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

[Paolo Bonzini]

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)? 
  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