On Wed, Mar 31, 2021, Paolo Bonzini wrote: > 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. 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. > 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? Ah, crud, yes. Holding a spinlock across the entire start() ... end() would be bad, especially when the notifier can block since that opens up the possibility of the task sleeping/blocking/yielding while the spinlock is held. Bummer.
