linux-kernel - Memory-ordering recipes

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Date:   Sun, 17 Sep 2017 16:05:09 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     j.alglave@....ac.uk, luc.maranget@...ia.fr, parri.andrea@...il.com,
        stern@...land.harvard.edu, dhowells@...hat.com,
        peterz@...radead.org, will.deacon@....com, boqun.feng@...il.com,
        npiggin@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: Memory-ordering recipes

Hello!

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
	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.

Others?

							Thanx, Paul