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

[Waiman Long <longman@redhat.com>]

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@redhat.com>
> ---
>   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?

Thanks,
Longman