Re: [PATCH v2 1/2] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 12, 2022 at 09:33:32AM -0400, Waiman Long wrote:
> A non-first waiter can potentially spin in the for loop of
> rwsem_down_write_slowpath() without sleeping but fail to acquire the
> lock even if the rwsem is free if the following sequence happens:
> 
>   Non-first waiter       First waiter      Lock holder
>   ----------------       ------------      -----------
>   Acquire wait_lock
>   rwsem_try_write_lock():
>     Set handoff bit if RT or
>       wait too long
>     Set waiter->handoff_set
>   Release wait_lock
>                          Acquire wait_lock
>                          Inherit waiter->handoff_set
>                          Release wait_lock
> 					   Clear owner
>                                            Release lock
>   if (waiter.handoff_set) {
>     rwsem_spin_on_owner(();
>     if (OWNER_NULL)
>       goto trylock_again;
>   }
>   trylock_again:
>   Acquire wait_lock
>   rwsem_try_write_lock():
>      if (first->handoff_set && (waiter != first))
>      	return false;
>   Release wait_lock
> 
> It is especially problematic if the non-first waiter is an RT task and
> it is running on the same CPU as the first waiter as this can lead to
> live lock.
> 

So why not do a better handoff? Specifically, have the owner set owner
to first-waiter instead of NULL ? (like the normal mutex code)