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
