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
