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
