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

[Paolo Bonzini]

On 31/03/21 21:47, Sean Christopherson wrote:
> Rereading things, a small chunk of the rwsem nastiness can go away.  I don't see
> any reason to use rw_semaphore instead of rwlock_t.

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.

I also thought of busy waiting on down_read_trylock if the MMU notifier 
cannot block, but that would also be invalid for the opposite reason 
(the down_write task might be asleep, waiting for other readers to 
release the task, and the down_read_trylock busy loop might not let that 
task run).

> And that's _already_ the worst case since notifications are currently
> serialized by mmu_lock.

But right now notifications are not a single critical section, they're 
two, aren't they?

Paolo