linux-kernel - Re: [PATCH v2] rwsem: Support optimistic spinning

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Message-ID: <20140429151100.GC8754@linux.vnet.ibm.com>
Date:	Tue, 29 Apr 2014 08:11:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Davidlohr Bueso <davidlohr@...com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning

On Mon, Apr 28, 2014 at 05:50:49PM -0700, Tim Chen wrote:
> On Mon, 2014-04-28 at 16:10 -0700, Paul E. McKenney wrote:
> 
> > > +#ifdef CONFIG_SMP
> > > +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > > +{
> > > +	int retval;
> > > +	struct task_struct *owner;
> > > +
> > > +	rcu_read_lock();
> > > +	owner = ACCESS_ONCE(sem->owner);
> > 
> > OK, I'll bite...
> > 
> > Why ACCESS_ONCE() instead of rcu_dereference()?
> 
> We're using it as a speculative check on the sem->owner to see
> if the owner is running on the cpu.  The rcu_read_lock
> is used for ensuring that the owner->on_cpu memory is
> still valid.

OK, so if we read complete garbage, all that happens is that we
lose a bit of performance?  If so, I am OK with it as long as there
is a comment (which Davidlohr suggested later in this thread).

							Thanx, Paul

> > (My first question was "where is the update side", but this is covered
> > by task_struct allocation and deallocation.)
> 
> Tim
> 

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