Memory-ordering recipes

From: Paul E. McKenney
Date: Sun Sep 17 2017 - 19:05:57 EST


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

Many of the examples are on-point, but this is aimed more
at understanding the memory model than at an organized set
of recipes.

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
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.


Thanx, Paul