linux-kernel - Re: [PATCH v4] locking/rwsem: Make handoff bit handling more consistent

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Date:   Mon, 15 Nov 2021 17:01:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        mazhenhua <mazhenhua@...omi.com>,
        Hillf Danton <hdanton@...a.com>,
        Maria Yu <quic_aiquny@...cinc.com>
Subject: Re: [PATCH v4] locking/rwsem: Make handoff bit handling more
 consistent

On Sun, Nov 14, 2021 at 10:38:57PM -0500, Waiman Long wrote:
> On 11/12/21 07:10, Peter Zijlstra wrote:

> > Argh, rwsem_mark_wake() doesn't clear HANDOFF when list_empty(), and
> > write_slowpath() is *far* too clever about all of this.
> rwsem_mark_wake() does clear the HANDOFF flag if it was set.

Argh, yeah, I got confused by the whole !woken case, but that case won't
ever hit list_empty() either. Perhaps that stuff could use a bit of a
reflow too.


> > > @@ -1098,7 +1110,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
> > >   		 * In this case, we attempt to acquire the lock again
> > >   		 * without sleeping.
> > >   		 */
> > > -		if (wstate == WRITER_HANDOFF) {
> > > +		if (waiter.handoff_set) {
> > I'm thinking this wants to be something like:
> > 
> > 		if (rwsem_first_waiter(sem) == &waiter && waiter.handoff_set) {
> handoff_set flag is only set when the waiter becomes the first.

Yes, but a random waiter can wake up and see it be set and also start
spinning.

> > 
> > >   			enum owner_state owner_state;
> > >   			preempt_disable();

> > @@ -575,6 +610,11 @@ static inline bool rwsem_try_write_lock(
> >   		return false;
> >   	}
> > +	/*
> > +	 * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
> > +	 * success.
> > +	 */
> > +	list_del(&waiter->list);
> >   	rwsem_set_owner(sem);
> >   	return true;
> >   }

> > @@ -1128,16 +1153,14 @@ rwsem_down_write_slowpath(struct rw_sema
> >   		raw_spin_lock_irq(&sem->wait_lock);
> >   	}
> >   	__set_current_state(TASK_RUNNING);
> > -	list_del(&waiter.list);
> +    rwsem_del_waiter(sem, &waiters); ?

I tried that, but then we get an extra atomic in this path. As is I made
try_write_lock() do the full del_waiter, see the hunk above.

> >   	raw_spin_unlock_irq(&sem->wait_lock);
> >   	lockevent_inc(rwsem_wlock);
> > -
> > -	return ret;
> > +	return sem;
> >   out_nolock:
> >   	__set_current_state(TASK_RUNNING);
> >   	raw_spin_lock_irq(&sem->wait_lock);
> > -	rwsem_out_nolock_clear_flags(sem, &waiter);
> > +	rwsem_del_waiter(sem, &waiter);
> >   	if (!list_empty(&sem->wait_list))
> >   		rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
> >   	raw_spin_unlock_irq(&sem->wait_lock);
> > 
> Sorry for the late reply as I was busy on other works.
> 
> I like the idea. I will incorporate in a new patch tomorrow.

Thanks!