Re: [PATCH] futex: replace bare barrier() with more lightweight READ_ONCE()

[Paul E. McKenney]

On Fri, Mar 04, 2016 at 01:05:24PM -0800, Darren Hart wrote:
> On Fri, Mar 04, 2016 at 09:12:31AM +0800, Jianyu Zhan wrote:
> > On Fri, Mar 4, 2016 at 1:05 AM, Darren Hart <dvhart@xxxxxxxxxxxxx> wrote:
> > > I thought I provided a corrected comment block.... maybe I didn't. We have been
> > > working on improving the futex documentation, so we're paying close attention to
> > > terminology as well as grammar. This one needs a couple minor tweaks. I suggest:
> > >
> > > /*
> > >  * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
> > >  * optimizing lock_ptr out of the logic below.
> > >  */
> > >
> > > The bit about q->lock_ptr possibly changing is already covered by the large
> > > comment block below the spin_lock(lock_ptr) call.
> > 
> > The large comment block is explaining the why the retry logic is required.
> > To achieve this semantic requirement,  the READ_ONCE is needed to prevent
> > compiler optimizing it by doing double loads.
> > 
> > So I think the comment above should explain this tricky part.
> 
> Fair point. Consider:
> 
> 
> /*
>  * q->lock_ptr can change between this read and the following spin_lock.
>  * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
>  * optimizing lock_ptr out of the logic below.
>  */
> 
> > 
> > > /* Use READ_ONCE to forbid the compiler from reloading q->lock_ptr  in spin_lock()  */
> > 
> > And as for preventing from optimizing the lock_ptr out of the retry
> > code block,  I have consult
> > Paul Mckenney,  he suggests one more  READ_ONCE should be added here:
> 
> Let's keep this discussion together so we have a record of the
> justification.
> 
> +Paul McKenney
> 
> Paul, my understanding was that spin_lock was a CPU memory barrier,
> which in turn is an implicit compiler barrier (aka barrier()), of which
> READ_ONCE is described as a weaker form. Reviewing this, I realize the
> scope of barrier() wasn't clear to me. It seems while barrier() ensures
> ordering, it does not offer the same guarantee regarding reloading that
> READ_ONCE offers. So READ_ONCE is not strictly a weaker form of
> barrier() as I had gathered from a spotty reading of
> memory-barriers.txt, but it also offers guarantees regarding memory
> references that barrier() does not.
> 
> Correct?

If q->lock_ptr is never changed except under that lock, then there is
indeed no reason for the ACCESS_ONCE().

So, is q->lock_ptr ever changed while the lock is -not- held?  If so,
I suggest that you put an ACCESS_ONCE() there.

								Thanx, Paul

> > if (unlikely(lock_ptr != READ_ONCE(q->lock_ptr))) {
> > <------------------------------
> >                   spin_unlock(lock_ptr);
> >                   goto retry;
> >  }
> > 
> > And I think this are two problem, and should be separated into two patches?
> 
> Yes (pending results of the conversation above).
> 
> -- 
> Darren Hart
> Intel Open Source Technology Center
>