linux-kernel - Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write

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Message-ID: <20160511093127.GI16677@dhcp22.suse.cz>
Date:	Wed, 11 May 2016 11:31:27 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <Waiman.Long@....com>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for
 down_write_killable

On Wed 11-05-16 11:17:33, Peter Zijlstra wrote:
> On Wed, May 11, 2016 at 11:04:42AM +0200, Michal Hocko wrote:
> > On Wed 11-05-16 10:44:01, Peter Zijlstra wrote:
> > [...]
> > > @@ -504,6 +502,18 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
> > >  	raw_spin_unlock_irq(&sem->wait_lock);
> > >  
> > >  	return ret;
> > > +
> > > +out_nolock:
> > > +	__set_current_state(TASK_RUNNING);
> > > +	raw_spin_lock_irq(&sem->wait_lock);
> > > +	list_del(&waiter.list);
> > > +	if (list_empty(&sem->wait_list))
> > > +		rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem);
> > > +	else
> > > +		__rwsem_do_wake(sem, RWSEM_WAKE_READERS);
> > > +	raw_spin_unlock_irq(&sem->wait_lock);
> > > +
> > > +	return ERR_PTR(-EINTR);
> > >  }
> > 
> > Looks much better but don't we have to check the count for potentially
> > pending writers?
> 
> Ah, so I was thinking that if we get here, there must still be an owner,
> otherwise we'd have acquired the lock. And if there is an owner, we
> cannot go wake writers. Hence the WAKE_READERS thing.

I was worried about the case where the owner is writer and we would wake
readers but I have missed that this wouldn't happen because of

	if (wake_type != RWSEM_WAKE_READ_OWNED) {
		adjustment = RWSEM_ACTIVE_READ_BIAS;
 try_reader_grant:
		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
			/* A writer stole the lock. Undo our reader grant. */
			if (rwsem_atomic_update(-adjustment, sem) &
						RWSEM_ACTIVE_MASK)
				goto out;
			/* Last active locker left. Retry waking readers. */
			goto try_reader_grant;
		}
	}

> Then again, WAKE_ANY would not harm, in that if we do wake a pending
> writer it will not proceed if it cannot and it'll go back to sleep
> again.

true

> So yeah, maybe WAKE_ANY is the prudent thing to do.

I guess so.

Care to cook up a full patch?

Thanks!
-- 
Michal Hocko
SUSE Labs