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SubjectRe: Memory-ordering recipes
On Mon, Sep 18, 2017 at 03:52:42PM +0800, Boqun Feng wrote:
> On Sun, Sep 17, 2017 at 04:05:09PM -0700, Paul E. McKenney wrote:
> > Hello!
> >
>
> Hi Paul,
>
> > The topic of memory-ordering recipes came up at the Linux Plumbers
> > Conference microconference on Friday, so I thought that I should summarize
> > what is currently "out there":
> >
> > 1. memory-barriers.txt: A bit rambling and diffuse for a recipes
> > document.
> >
> > 2. https://www.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/Examples.html
> > Many of the examples are on-point, but this is aimed more
> > at understanding the memory model than at an organized set
> > of recipes.
> >
> > 3. https://www.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/Examples.html
>
> Duplicate links ;-) This should a link to some slides?

Indeed! How about this one?

http://www.linuxplumbersconf.org/2017/ocw//system/presentations/4708/original/LKMM-overview.2017.09.15b.pdf

> > Slides 15-20. Again, some of the litmus tests are on-point,
> > but the focus is more on understanding the memory model than on
> > an organized set of recipes.
> >
> > So what litmus tests are needed? Here is my initial set:
> >
> > 1. Release-acquire chains, AKA ISA2, Z6.2, LB, and 3.LB
> >
> > Lots of variety here, can in some cases substitute:
> >
> > a. READ_ONCE() for smp_load_acquire()
> > b. WRITE_ONCE() for smp_store_release()
> > c. Dependencies for both smp_load_acquire() and
> > smp_store_release().
> > d. smp_wmb() for smp_store_release() in first thread
> > of ISA2 and Z6.2.
> > e. smp_rmb() for smp_load_acquire() in last thread of ISA2.
> >
> > 2. MP (see test6.pdf for nickname translation)
> >
> > a. smp_store_release() / smp_load_acquire()
> > b. rcu_assign_pointer() / rcu_dereference()
> > c. smp_wmb() / smp_rmb()
> > d. Replacing either of the above with smp_mb()
> >
> > 3. SB
> >
> > a. smp_mb(), as in lockless wait-wakeup coordination.
> > And as in sys_membarrier()-scheduler coordination,
> > for that matter.
>
> b. replace smp_mb() with smp_mb__before_atomic() followed
> by a _relaxed cmpchg? As in pv_kick_node():
>
> https://marc.info/?l=linux-kernel&m=150274124711012
>
> Besides, do we also want to add Co* into the set? I think there may be
> some people still confused to think per-loc SC is not held, and they may
> add unnecessary barriers in their code. Those (Co*) recipes could serve
> as a guide for state-machine style programming. Thoughts?

Indeed, it would be good to have some single-variable-SC recipes.

And single-variable-SC holds only if you use READ_ONCE(). ;-)

Thanx, Paul

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