Re: wake_q memory ordering

[Manfred Spraul]

Hi Peter,

On 10/10/19 1:42 PM, Peter Zijlstra wrote:
> On Thu, Oct 10, 2019 at 12:41:11PM +0200, Manfred Spraul wrote:
> > Hi,
> >
> > Waiman Long noticed that the memory barriers in sem_lock() are not really
> > documented, and while adding documentation, I ended up with one case where
> > I'm not certain about the wake_q code:
> >
> > Questions:
> > - Does smp_mb__before_atomic() + a (failed) cmpxchg_relaxed provide an
> >  Â ordering guarantee?
> Yep. Either the atomic instruction implies ordering (eg. x86 LOCK
> prefix) or it doesn't (most RISC LL/SC), if it does,
> smp_mb__{before,after}_atomic() are a NO-OP and the ordering is
> unconditinoal, if it does not, then smp_mb__{before,after}_atomic() are
> unconditional barriers.

And _relaxed() differs from "normal" cmpxchg only for LL/SC 
architectures, correct?

Therefore smp_mb__{before,after}_atomic() may be combined with 
cmpxchg_relaxed, to form a full memory barrier, on all archs.

[...]


> > - Is it ok that wake_up_q just writes wake_q->next, shouldn't
> >  Â smp_store_acquire() be used? I.e.: guarantee that wake_up_process()
> >  Â happens after cmpxchg_relaxed(), assuming that a failed cmpxchg_relaxed
> >  Â provides any ordering.
> There is no such thing as store_acquire, it is either load_acquire or
> store_release. But just like how we can write load-aquire like
> load+smp_mb(), so too I suppose we could write store-acquire like
> store+smp_mb(), and that is exactly what is there (through the implied
> barrier of wake_up_process()).

Thanks for confirming my assumption:
The code is correct, due to the implied barrier inside wake_up_process().

[...]
> > rewritten:
> >
> > start condition: A = 1; B = 0;
> >
> > CPU1:
> >  ÂÂÂ B = 1;
> >  ÂÂÂ RELEASE, unlock LockX;
> >
> > CPU2:
> >  ÂÂÂ lock LockX, ACQUIRE
> >  ÂÂÂ if (LOAD A == 1) return; /* using cmp_xchg_relaxed */
> >
> > CPU2:
> >  ÂÂÂ A = 0;
> >  ÂÂÂ ACQUIRE, lock LockY
> >  ÂÂÂ smp_mb__after_spinlock();
> >  ÂÂÂ READ B
> >
> > Question: is A = 1, B = 0 possible?
> Your example is incomplete (there is no A=1 assignment for example), but
> I'm thinking I can guess where that should go given the earlier text.

A=1 is listed as start condition. Way before, someone did wake_q_add().


> I don't think this is broken.

Thanks.

--

ÂÂÂ Manfred