Messages in this thread | | | Date | Fri, 4 Mar 2016 14:38:01 -0800 | From | Darren Hart <> | Subject | Re: [PATCH] futex: replace bare barrier() with more lightweight READ_ONCE() |
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On Fri, Mar 04, 2016 at 01:57:20PM -0800, Paul McKenney wrote: > 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@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 q->lock_ptr is never changed except under that lock, then there is > indeed no reason for the ACCESS_ONCE().
The only location where a q->lock_ptr is updated without that lock being held is in queue_lock(). This is safe as the futex_q is not yet queued onto an hb until after the lock is held (so unqueue_me() cannot race with queue_lock()).
> So, is q->lock_ptr ever changed while the lock is -not- held? If so, > I suggest that you put an ACCESS_ONCE() there.
It is not.
-- Darren Hart Intel Open Source Technology Center
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