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SubjectRe: Memory-ordering recipes
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?

> 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?

Regards,
Boqun

>
> Others?
>
> Thanx, Paul
>
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