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

[Waiman Long]

On 10/13/22 09:33, Waiman Long wrote:
> On 10/13/22 06:02, Peter Zijlstra wrote:
> > 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)
>
> I understand your desire to make the rwsem handoff process more like 
> what mutex is currently doing. I certainly think it is doable and will 
> put this in my todo list. However, that needs to be done at unlock and 
> wakeup time. I expect that will require moderate amount of code 
> changes which will make it not that suitable for backporting to the 
> stable releases.
>
> I would like to see these simple fixes get merged first and then we 
> can work on a major revamp of the handoff code. What do you think?
I am planning to post additional patches on top to rework the handoff 
code sometimes next week, but I will keep these fix patches for the 
stable releases.

Cheers,
Longman