linux-kernel - Re: Memory-ordering recipes

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Date:   Mon, 18 Sep 2017 07:25:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     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, npiggin@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: 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