Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already set

[Waiman Long]

On 11/11/21 16:01, Waiman Long wrote:
> On 11/11/21 15:26, Peter Zijlstra wrote:
> > On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
> >
> > > @@ -434,6 +430,7 @@ static void rwsem_mark_wake(struct rw_semaphore 
> > > *sem,
> > >               if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
> > >                   time_after(jiffies, waiter->timeout)) {
> > >                   adjustment -= RWSEM_FLAG_HANDOFF;
> > > +                waiter->handoff_set = true;
> > >                   lockevent_inc(rwsem_rlock_handoff);
> > >               }
> > Do we really need this flag? Wouldn't it be the same as waiter-is-first
> > AND sem-has-handoff ?
> That is true. The only downside is that we have to read the count 
> first in rwsem_out_nolock_clear_flags(). Since this is not a fast 
> path, it should be OK to do that.

I just realize that I may still need this flag for writer to determine 
if it should spin after failing to acquire the lock. Or I will have to 
do extra read of count value in the loop. I don't need to use it for 
writer now.

Cheers,
Longman