On Fri, Mar 04, 2016 at 09:12:31AM +0800, Jianyu Zhan wrote:
> On Fri, Mar 4, 2016 at 1:05 AM, Darren Hart <dvh...@infradead.org> 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 (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
