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

[Mukesh Ojha]

Hi,

On 9/29/2022 11:36 PM, Waiman Long wrote:
> On 9/29/22 14:04, 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.
> >
> > Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more 
> > consistent")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   kernel/locking/rwsem.c | 13 ++++++++++---
> >   1 file changed, 10 insertions(+), 3 deletions(-)
>
> Mukesh, can you test if this patch can fix the RT task lockup problem?

I will come back with the result.

Thank you.

> Thanks,
> Longman