Re: [PATCH v6 0/6] lockinig/rwsem: Fix rwsem bugs & enable true lock handoff

[Waiman Long]

On 1/22/23 22:40, Waiman Long wrote:
> On 1/22/23 08:46, Peter Zijlstra wrote:
> > On Thu, Nov 17, 2022 at 09:20:10PM -0500, Waiman Long wrote:
> >
> > > Waiman Long (6):
> > >    locking/rwsem: Prevent non-first waiter from spinning in 
> > > down_write() slowpath
> > >    locking/rwsem: Disable preemption at all down_read*() and 
> > > up_read() code paths
> > >    locking/rwsem: Disable preemption at all down_write*() and 
> > > up_write() code paths
> > >    locking/rwsem: Change waiter->hanodff_set to a handoff_state enum
> > After all these we still have down_read_non_owner() using
> > __rwsem_set_reader_owner() outside of the preempt_disable() region.
> >
> > Now, let me go stare at this one:
>
> Thanks for spotting that. I kind of overlook the fact we have a 
> down_read_non_owner(). Will update that as well.

After looking further into this, we will need to either pass one more 
parameter to __down_read() for this special case or to do 
preempt_disable/enable in each of the higher level down_read functions. 
As down_read_non_owner() is a rarely called function, I doubt it is 
worth the extra effort to do that since the owner value (other than the 
RWSEM_READER_OWNED bit) is for debugging purpose only and is not 
critical to the correct functioning of a rwsem. I will add a comment to 
note that.

Cheers,
Longman