Re: [PATCH 3/6] locking/rwsem: Rework writer wakeup

[Peter Zijlstra]

On Thu, Feb 23, 2023 at 04:38:08PM -0500, Waiman Long wrote:

> > @@ -1143,54 +1138,36 @@ rwsem_down_write_slowpath(struct rw_sema
> >   	} else {
> >   		atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
> >   	}
> > +	raw_spin_unlock_irq(&sem->wait_lock);
> >   	/* wait until we successfully acquire the lock */
> > -	set_current_state(state);
> >   	trace_contention_begin(sem, LCB_F_WRITE);
> >   	for (;;) {
> > -		if (rwsem_try_write_lock(sem, &waiter)) {
> > -			/* rwsem_try_write_lock() implies ACQUIRE on success */
> > +		set_current_state(state);
> > +		if (!smp_load_acquire(&waiter.task)) {
> > +			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
> >   			break;
> >   		}
> > -
> > -		raw_spin_unlock_irq(&sem->wait_lock);
> > -
> > -		if (signal_pending_state(state, current))
> > -			goto out_nolock;
> > -
> > -		/*
> > -		 * After setting the handoff bit and failing to acquire
> > -		 * the lock, attempt to spin on owner to accelerate lock
> > -		 * transfer. If the previous owner is a on-cpu writer and it
> > -		 * has just released the lock, OWNER_NULL will be returned.
> > -		 * In this case, we attempt to acquire the lock again
> > -		 * without sleeping.
> > -		 */
> > -		if (waiter.handoff_set) {
> > -			enum owner_state owner_state;
> > -
> > -			owner_state = rwsem_spin_on_owner(sem);
> > -			if (owner_state == OWNER_NULL)
> > -				goto trylock_again;
> > +		if (signal_pending_state(state, current)) {
> > +			raw_spin_lock_irq(&sem->wait_lock);
> > +			if (waiter.task)
> > +				goto out_nolock;
> > +			raw_spin_unlock_irq(&sem->wait_lock);
> > +			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
> > +			break;
> >   		}
> > -
> >   		schedule_preempt_disabled();
> >   		lockevent_inc(rwsem_sleep_writer);
> > -		set_current_state(state);
> > -trylock_again:
> > -		raw_spin_lock_irq(&sem->wait_lock);
> >   	}
> >   	__set_current_state(TASK_RUNNING);
> > -	raw_spin_unlock_irq(&sem->wait_lock);
> >   	lockevent_inc(rwsem_wlock);
> >   	trace_contention_end(sem, 0);
> >   	return sem;
> >   out_nolock:
> > -	__set_current_state(TASK_RUNNING);
> > -	raw_spin_lock_irq(&sem->wait_lock);
> >   	rwsem_del_wake_waiter(sem, &waiter, &wake_q);
> > +	__set_current_state(TASK_RUNNING);
> >   	lockevent_inc(rwsem_wlock_fail);
> >   	trace_contention_end(sem, -EINTR);
> >   	return ERR_PTR(-EINTR);
> 
> I believe it is better to change state inside the wait_lock critical section
> to provide a release barrier for free.

I can't follow... a release for what? Note that the reader slowpath has
this exact form already.