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Date:	Mon, 16 May 2016 05:17:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Waiman Long <Waiman.Long@....com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>, kcc@...gle.com,
	dvyukov@...gle.com
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

On Mon, May 16, 2016 at 01:09:48PM +0200, Peter Zijlstra wrote:
> On Fri, May 13, 2016 at 10:58:05AM -0700, Peter Hurley wrote:
> > > Note that barrier() and READ_ONCE() have overlapping but not identical
> > > results and the combined use actually makes sense here.
> > > 
> > > Yes, a barrier() anywhere in the loop will force a reload of the
> > > variable, _however_ it doesn't force that reload to not suffer from
> > > load tearing.
> > > 
> > > Using volatile also forces a reload, but also ensures the load cannot
> > > be torn IFF it is of machine word side and naturally aligned.
> > > 
> > > So while the READ_ONCE() here is pointless for forcing the reload;
> > > that's already ensured, we still need to make sure the load isn't torn.
> > 
> > If load tearing a naturally aligned pointer is a real code generation
> > possibility then the rcu list code is broken too (which loads ->next
> > directly; cf. list_for_each_entry_rcu() & list_for_each_entry_lockless()).
> > 
> > For 4.4, Paul added READ_ONCE() checks for list_empty() et al, but iirc
> > that had to do with control dependencies and not load tearing.
> 
> Well, Paul is the one who started the whole load/store tearing thing, so
> I suppose he knows what he's doing.

That had to do with suppressing false positives for one of Dmitry
Vjukov's concurrency checkers.  I suspect that Peter Hurley is right
that continued use of that checker would identify other places needing
READ_ONCE(), but from what I understand that is on hold pending a formal
definition of the Linux-kernel memory model.  (KCC and Dmitry (CCed)
can correct my if I am confused on this point.)

> That said; its a fairly recent as things go so lots of code hasn't been
> updated yet, and its also a very unlikely thing for a compiler to do;
> since it mostly doesn't make sense to emit multiple instructions where
> one will do, so its not a very high priority thing either.
> 
> But from what I understand, the compiler is free to emit all kinds of
> nonsense for !volatile loads/stores.

That is quite true.  :-/

> > OTOH, this patch might actually produce store-tearing:
> > 
> > +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
> > +{
> > +	/*
> > +	 * We check the owner value first to make sure that we will only
> > +	 * do a write to the rwsem cacheline when it is really necessary
> > +	 * to minimize cacheline contention.
> > +	 */
> > +	if (sem->owner != RWSEM_READER_OWNED)
> > +		sem->owner = RWSEM_READER_OWNED;
> > +}
> 
> Correct; which is why we should always use {READ,WRITE}_ONCE() for
> anything that is used locklessly.

Completely agreed.  Improve readability of code by flagging lockless
shared-memory accesses, help checkers better find bugs, and prevent the
occasional compiler mischief!

							Thanx, Paul

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