Automate memory-barriers.txt; provide Linux-kernel memory model

                                  Prior Operation     Subsequent Operation

                                  ---------------  ---------------------------

                               C  Self  R  W  RWM  Self  R  W  DR  DW  RMW  SV

                              __  ----  -  -  ---  ----  -  -  --  --  ---  --

Store, e.g., WRITE_ONCE()            Y                                       Y

Load, e.g., READ_ONCE()              Y                              Y        Y

Unsuccessful RMW operation           Y                              Y        Y

smp_read_barrier_depends()              Y                       Y   Y

*_dereference()                      Y                          Y   Y        Y

Successful *_acquire()               R                   Y  Y   Y   Y    Y   Y

Successful *_release()         C        Y  Y    Y     W                      Y

smp_rmb()                               Y       R        Y      Y        R

smp_wmb()                                  Y    W           Y       Y    W

smp_mb() & synchronize_rcu()  CP        Y  Y    Y        Y  Y   Y   Y    Y

Successful full non-void RMW  CP     Y  Y  Y    Y     Y  Y  Y   Y   Y    Y   Y

smp_mb__before_atomic()       CP        Y  Y    Y        a  a   a   a    Y

smp_mb__after_atomic()        CP        a  a    Y        Y  Y   Y   Y

Key:	C:	Ordering is cumulative

	P:	Ordering propagates

	R:	Read, for example, READ_ONCE(), or read portion of RMW

	W:	Write, for example, WRITE_ONCE(), or write portion of RMW

	Y:	Provides ordering

	a:	Provides ordering given intervening RMW atomic operation

	DR:	Dependent read (address dependency)

	DW:	Dependent write (address, data, or control dependency)

	RMW:	Atomic read-modify-write operation

	SV	Same-variable access

This document provides "recipes", that is, litmus tests for commonly

occurring situations, as well as a few that illustrate subtly broken but

attractive nuisances.  Many of these recipes include example code from

v4.13 of the Linux kernel.

The first section covers simple special cases, the second section

takes off the training wheels to cover more involved examples,

and the third section provides a few rules of thumb.

Simple special cases

====================

This section presents two simple special cases, the first being where

there is only one CPU or only one memory location is accessed, and the

second being use of that old concurrency workhorse, locking.

Single CPU or single memory location

------------------------------------

If there is only one CPU on the one hand or only one variable

on the other, the code will execute in order.  There are (as

usual) some things to be careful of:

1.	Some aspects of the C language are unordered.  For example,

	in the expression "f(x) + g(y)", the order in which f and g are

	called is not defined; the object code is allowed to use either

	order or even to interleave the computations.

2.	Compilers are permitted to use the "as-if" rule.  That is, a

	compiler can emit whatever code it likes for normal accesses,

	as long as the results of a single-threaded execution appear

	just as if the compiler had followed all the relevant rules.

	To see this, compile with a high level of optimization and run

	the debugger on the resulting binary.

3.	If there is only one variable but multiple CPUs, that variable

	must be properly aligned and all accesses to that variable must

	be full sized.	Variables that straddle cachelines or pages void

	your full-ordering warranty, as do undersized accesses that load

	from or store to only part of the variable.

4.	If there are multiple CPUs, accesses to shared variables should

	use READ_ONCE() and WRITE_ONCE() or stronger to prevent load/store

	tearing, load/store fusing, and invented loads and stores.

	There are exceptions to this rule, including:

	i.	When there is no possibility of a given shared variable

		being updated by some other CPU, for example, while

		holding the update-side lock, reads from that variable

		need not use READ_ONCE().

	ii.	When there is no possibility of a given shared variable

		being either read or updated by other CPUs, for example,

		when running during early boot, reads from that variable

		need not use READ_ONCE() and writes to that variable

		need not use WRITE_ONCE().

Locking

-------

Locking is well-known and straightforward, at least if you don't think

about it too hard.  And the basic rule is indeed quite simple: Any CPU that

has acquired a given lock sees any changes previously seen or made by any

CPU before it released that same lock.  Note that this statement is a bit

stronger than "Any CPU holding a given lock sees all changes made by any

CPU during the time that CPU was holding this same lock".  For example,

consider the following pair of code fragments:

	/* See MP+polocks.litmus. */

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		spin_lock(&mylock);

		WRITE_ONCE(y, 1);

		spin_unlock(&mylock);

	}

	void CPU1(void)

	{

		spin_lock(&mylock);

		r0 = READ_ONCE(y);

		spin_unlock(&mylock);

		r1 = READ_ONCE(x);

	}

The basic rule guarantees that if CPU0() acquires mylock before CPU1(),

then both r0 and r1 must be set to the value 1.  This also has the

consequence that if the final value of r0 is equal to 1, then the final

value of r1 must also be equal to 1.  In contrast, the weaker rule would

say nothing about the final value of r1.

The converse to the basic rule also holds, as illustrated by the

following litmus test:

	/* See MP+porevlocks.litmus. */

	void CPU0(void)

	{

		r0 = READ_ONCE(y);

		spin_lock(&mylock);

		r1 = READ_ONCE(x);

		spin_unlock(&mylock);

	}

	void CPU1(void)

	{

		spin_lock(&mylock);

		WRITE_ONCE(x, 1);

		spin_unlock(&mylock);

		WRITE_ONCE(y, 1);

	}

This converse to the basic rule guarantees that if CPU0() acquires

mylock before CPU1(), then both r0 and r1 must be set to the value 0.

This also has the consequence that if the final value of r1 is equal

to 0, then the final value of r0 must also be equal to 0.  In contrast,

the weaker rule would say nothing about the final value of r0.

These examples show only a single pair of CPUs, but the effects of the

locking basic rule extend across multiple acquisitions of a given lock

across multiple CPUs.

However, it is not necessarily the case that accesses ordered by

locking will be seen as ordered by CPUs not holding that lock.

Consider this example:

	/* See Z6.0+pooncelock+pooncelock+pombonce.litmus. */

	void CPU0(void)

	{

		spin_lock(&mylock);

		WRITE_ONCE(x, 1);

		WRITE_ONCE(y, 1);

		spin_unlock(&mylock);

	}

	void CPU1(void)

	{

		spin_lock(&mylock);

		r0 = READ_ONCE(y);

		WRITE_ONCE(z, 1);

		spin_unlock(&mylock);

	}

	void CPU2(void)

	{

		WRITE_ONCE(z, 2);

		smp_mb();

		r1 = READ_ONCE(x);

	}

Counter-intuitive though it might be, it is quite possible to have

the final value of r0 be 1, the final value of z be 2, and the final

value of r1 be 0.  The reason for this surprising outcome is that

CPU2() never acquired the lock, and thus did not benefit from the

lock's ordering properties.

Ordering can be extended to CPUs not holding the lock by careful use

of smp_mb__after_spinlock():

	/* See Z6.0+pooncelock+poonceLock+pombonce.litmus. */

	void CPU0(void)

	{

		spin_lock(&mylock);

		WRITE_ONCE(x, 1);

		WRITE_ONCE(y, 1);

		spin_unlock(&mylock);

	}

	void CPU1(void)

	{

		spin_lock(&mylock);

		smp_mb__after_spinlock();

		r0 = READ_ONCE(y);

		WRITE_ONCE(z, 1);

		spin_unlock(&mylock);

	}

	void CPU2(void)

	{

		WRITE_ONCE(z, 2);

		smp_mb();

		r1 = READ_ONCE(x);

	}

This addition of smp_mb__after_spinlock() strengthens the lock acquisition

sufficiently to rule out the counter-intuitive outcome.

Taking off the training wheels

==============================

This section looks at more complex examples, including message passing,

load buffering, release-acquire chains, store buffering.

Many classes of litmus tests have abbreviated names, which may be found

here: https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf

Message passing (MP)

--------------------

The MP pattern has one CPU execute a pair of stores to a pair of variables

and another CPU execute a pair of loads from this same pair of variables,

but in the opposite order.  The goal is to avoid the counter-intuitive

outcome in which the first load sees the value written by the second store

but the second load does not see the value written by the first store.

In the absence of any ordering, this goal may not be met, as can be seen

in the MP+poonceonces.litmus litmus test.  This section therefore looks at

a number of ways of meeting this goal.

Release and acquire

~~~~~~~~~~~~~~~~~~~

Use of smp_store_release() and smp_load_acquire() is one way to force

the desired MP ordering.  The general approach is shown below:

	/* See MP+pooncerelease+poacquireonce.litmus. */

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		smp_store_release(&y, 1);

	}

	void CPU1(void)

	{

		r0 = smp_load_acquire(&y);

		r1 = READ_ONCE(x);

	}

The smp_store_release() macro orders any prior accesses against the

store, while the smp_load_acquire macro orders the load against any

subsequent accesses.  Therefore, if the final value of r0 is the value 1,

the final value of r1 must also be the value 1.

The init_stack_slab() function in lib/stackdepot.c uses release-acquire

in this way to safely initialize of a slab of the stack.  Working out

the mutual-exclusion design is left as an exercise for the reader.

Assign and dereference

~~~~~~~~~~~~~~~~~~~~~~

Use of rcu_assign_pointer() and rcu_dereference() is quite similar to the

use of smp_store_release() and smp_load_acquire(), except that both

rcu_assign_pointer() and rcu_dereference() operate on RCU-protected

pointers.  The general approach is shown below:

	/* See MP+onceassign+derefonce.litmus. */

	int z;

	int *y = &z;

	int x;

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		rcu_assign_pointer(y, &x);

	}

	void CPU1(void)

	{

		rcu_read_lock();

		r0 = rcu_dereference(y);

		r1 = READ_ONCE(*r0);

		rcu_read_unlock();

	}

In this example, if the final value of r0 is &x then the final value of

r1 must be 1.

The rcu_assign_pointer() macro has the same ordering properties as does

smp_store_release(), but the rcu_dereference() macro orders the load only

against later accesses that depend on the value loaded.  A dependency

is present if the value loaded determines the address of a later access

(address dependency, as shown above), the value written by a later store

(data dependency), or whether or not a later store is executed in the

first place (control dependency).  Note that the term "data dependency"

is sometimes casually used to cover both address and data dependencies.

In lib/prime_numbers.c, the expand_to_next_prime() function invokes

rcu_assign_pointer(), and the next_prime_number() function invokes

rcu_dereference().  This combination mediates access to a bit vector

that is expanded as additional primes are needed.

Write and read memory barriers

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is usually better to use smp_store_release() instead of smp_wmb()

and to use smp_load_acquire() instead of smp_rmb().  However, the older

smp_wmb() and smp_rmb() APIs are still heavily used, so it is important

to understand their use cases.  The general approach is shown below:

	/* See MP+wmbonceonce+rmbonceonce.litmus. */

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		smp_wmb();

		WRITE_ONCE(y, 1);

	}

	void CPU1(void)

	{

		r0 = READ_ONCE(y);

		smp_rmb();

		r1 = READ_ONCE(x);

	}

The smp_wmb() macro orders prior stores against later stores, and the

smp_rmb() macro orders prior loads against later loads.  Therefore, if

the final value of r0 is 1, the final value of r1 must also be 1.

The the xlog_state_switch_iclogs() function in fs/xfs/xfs_log.c contains

the following write-side code fragment:

	log->l_curr_block -= log->l_logBBsize;

	ASSERT(log->l_curr_block >= 0);

	smp_wmb();

	log->l_curr_cycle++;

And the xlog_valid_lsn() function in fs/xfs/xfs_log_priv.h contains

the corresponding read-side code fragment:

	cur_cycle = ACCESS_ONCE(log->l_curr_cycle);

	smp_rmb();

	cur_block = ACCESS_ONCE(log->l_curr_block);

Alternatively, consider the following comment in function

perf_output_put_handle() in kernel/events/ring_buffer.c:

	 *   kernel				user

	 *

	 *   if (LOAD ->data_tail) {		LOAD ->data_head

	 *			(A)		smp_rmb()	(C)

	 *	STORE $data			LOAD $data

	 *	smp_wmb()	(B)		smp_mb()	(D)

	 *	STORE ->data_head		STORE ->data_tail

	 *   }

The B/C pairing is an example of the MP pattern using smp_wmb() on the

write side and smp_rmb() on the read side.

Of course, given that smp_mb() is strictly stronger than either smp_wmb()

or smp_rmb(), any code fragment that would work with smp_rmb() and

smp_wmb() would also work with smp_mb() replacing either or both of the

weaker barriers.

Load buffering (LB)

-------------------

The LB pattern has one CPU load from one variable and then store to a

second, while another CPU loads from the second variable and then stores

to the first.  The goal is to avoid the counter-intuitive situation where

each load reads the value written by the other CPU's store.  In the

absence of any ordering it is quite possible that this may happen, as

can be seen in the LB+poonceonces.litmus litmus test.

One way of avoiding the counter-intuitive outcome is through the use of a

control dependency paired with a full memory barrier:

	/* See LB+ctrlonceonce+mbonceonce.litmus. */

	void CPU0(void)

	{

		r0 = READ_ONCE(x);

		if (r0)

			WRITE_ONCE(y, 1);

	}

	void CPU1(void)

	{

		r1 = READ_ONCE(y);

		smp_mb();

		WRITE_ONCE(x, 1);

	}

This pairing of a control dependency in CPU0() with a full memory

barrier in CPU1() prevents r0 and r1 from both ending up equal to 1.

The A/D pairing from the ring-buffer use case shown earlier also

illustrates LB.  Here is a repeat of the comment in

perf_output_put_handle() in kernel/events/ring_buffer.c, showing a

control dependency on the kernel side and a full memory barrier on

the user side:

	 *   kernel				user

	 *

	 *   if (LOAD ->data_tail) {		LOAD ->data_head

	 *			(A)		smp_rmb()	(C)

	 *	STORE $data			LOAD $data

	 *	smp_wmb()	(B)		smp_mb()	(D)

	 *	STORE ->data_head		STORE ->data_tail

	 *   }

	 *

	 * Where A pairs with D, and B pairs with C.

The kernel's control dependency between the load from ->data_tail

and the store to data combined with the user's full memory barrier

between the load from data and the store to ->data_tail prevents

the counter-intuitive outcome where the kernel overwrites the data

before the user gets done loading it.

Release-acquire chains

----------------------

Release-acquire chains are a low-overhead, flexible, and easy-to-use

method of maintaining order.  However, they do have some limitations that

need to be fully understood.  Here is an example that maintains order:

	/* See ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus. */

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		smp_store_release(&y, 1);

	}

	void CPU1(void)

	{

		r0 = smp_load_acquire(y);

		smp_store_release(&z, 1);

	}

	void CPU2(void)

	{

		r1 = smp_load_acquire(z);

		r2 = READ_ONCE(x);

	}

In this case, if r0 and r1 both have final values of 1, then r2 must

also have a final value of 1.

The ordering in this example is stronger than it needs to be.  For

example, ordering would still be preserved if CPU1()'s smp_load_acquire()

invocation was replaced with READ_ONCE().

It is tempting to assume that CPU0()'s store to x is globally ordered

before CPU1()'s store to z, but this is not the case:

	/* See Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus. */

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		smp_store_release(&y, 1);

	}

	void CPU1(void)

	{

		r0 = smp_load_acquire(y);

		smp_store_release(&z, 1);

	}

	void CPU2(void)

	{

		WRITE_ONCE(z, 2);

		smp_mb();

		r1 = READ_ONCE(x);

	}

One might hope that if the final value of r0 is 1 and the final value

of z is 2, then the final value of r1 must also be 1, but it really is

possible for r1 to have the final value of 0.  The reason, of course,

is that in this version, CPU2() is not part of the release-acquire chain.

This situation is accounted for in the rules of thumb below.

Despite this limitation, release-acquire chains are low-overhead as

well as simple and powerful, at least as memory-ordering mechanisms go.

Store buffering

---------------

Store buffering can be thought of as upside-down load buffering, so

that one CPU first stores to one variable and then loads from a second,

while another CPU stores to the second variable and then loads from the

first.  Preserving order requires nothing less than full barriers:

	/* See SB+mbonceonces.litmus. */

	void CPU0(void)

	{

		WRITE_ONCE(x, 1);

		smp_mb();

		r0 = READ_ONCE(y);

	}

	void CPU1(void)

	{

		WRITE_ONCE(y, 1);

		smp_mb();

		r1 = READ_ONCE(x);

	}

Omitting either smp_mb() will allow both r0 and r1 to have final

values of 0, but providing both full barriers as shown above prevents

this counter-intuitive outcome.

This pattern most famously appears as part of Dekker's locking

algorithm, but it has a much more practical use within the Linux kernel

of ordering wakeups.  The following comment taken from waitqueue_active()

in include/linux/wait.h shows the canonical pattern:

 *      CPU0 - waker                    CPU1 - waiter

 *

 *                                      for (;;) {

 *      @cond = true;                     prepare_to_wait(&wq_head, &wait, state);

 *      smp_mb();                         // smp_mb() from set_current_state()

 *      if (waitqueue_active(wq_head))         if (@cond)

 *        wake_up(wq_head);                      break;

 *                                        schedule();

 *                                      }

 *                                      finish_wait(&wq_head, &wait);

On CPU0, the store is to @cond and the load is in waitqueue_active().

On CPU1, prepare_to_wait() contains both a store to wq_head and a call

to set_current_state(), which contains an smp_mb() barrier; the load is

"if (@cond)".  The full barriers prevent the undesirable outcome where

CPU1 puts the waiting task to sleep and CPU0 fails to wake it up.

Note that use of locking can greatly simplify this pattern.

Rules of thumb

==============

There might seem to be no pattern governing what ordering primitives are

needed in which situations, but this is not the case.  There is a pattern

based on the relation between the accesses linking successive CPUs in a

given litmus test.  There are three types of linkage:

1.	Write-to-read, where the next CPU reads the value that the

	previous CPU wrote.  The LB litmus-test patterns contain only

	this type of relation.	In formal memory-modeling texts, this

	relation is called "reads-from" and is usually abbreviated "rf".

2.	Read-to-write, where the next CPU overwrites the value that the

	previous CPU read.  The SB litmus test contains only this type

	of relation.  In formal memory-modeling texts, this relation is

	often called "from-reads" and is sometimes abbreviated "fr".

3.	Write-to-write, where the next CPU overwrites the value written

	by the previous CPU.  The Z6.0 litmus test pattern contains a

	write-to-write relation between the last access of CPU1() and

	the first access of CPU2().  In formal memory-modeling texts,

	this relation is often called "coherence order" and is sometimes

	abbreviated "co".  In the C++ standard, it is instead called

	"modification order" and often abbreviated "mo".

The strength of memory ordering required for a given litmus test to

avoid a counter-intuitive outcome depends on the types of relations

linking the memory accesses for the outcome in question:

o	If all links are write-to-read links, then the weakest

	possible ordering within each CPU suffices.  For example, in

	the LB litmus test, a control dependency was enough to do the

	job.

o	If all but one of the links are write-to-read links, then a

	release-acquire chain suffices.  Both the MP and the ISA2

	litmus tests illustrate this case.

o	If more than one of the links are something other than

	write-to-read links, then a full memory barrier is required

	between each successive pair of non-write-to-read links.  This

	case is illustrated by the Z6.0 litmus tests, both in the

	locking and in the release-acquire sections.

However, if you find yourself having to stretch these rules of thumb

to fit your situation, you should consider creating a litmus test and

running it on the model.

This document provides background reading for memory models and related

tools.  These documents are aimed at kernel hackers who are interested

in memory models.

Hardware manuals and models

===========================

o	SPARC International Inc. (Ed.). 1994. "The SPARC Architecture

	Reference Manual Version 9". SPARC International Inc.

o	Compaq Computer Corporation (Ed.). 2002. "Alpha Architecture

	Reference Manual".  Compaq Computer Corporation.

o	Intel Corporation (Ed.). 2002. "A Formal Specification of Intel

	Itanium Processor Family Memory Ordering". Intel Corporation.

o	Intel Corporation (Ed.). 2002. "Intel 64 and IA-32 Architectures

	Software Developer’s Manual". Intel Corporation.

o	Peter Sewell, Susmit Sarkar, Scott Owens, Francesco Zappa Nardelli,

	and Magnus O. Myreen. 2010. "x86-TSO: A Rigorous and Usable

	Programmer's Model for x86 Multiprocessors". Commun. ACM 53, 7

	(July, 2010), 89-97. http://doi.acm.org/10.1145/1785414.1785443

o	IBM Corporation (Ed.). 2009. "Power ISA Version 2.06". IBM

	Corporation.

o	ARM Ltd. (Ed.). 2009. "ARM Barrier Litmus Tests and Cookbook".

	ARM Ltd.

o	Susmit Sarkar, Peter Sewell, Jade Alglave, Luc Maranget, and

	Derek Williams.  2011. "Understanding POWER Multiprocessors". In

	Proceedings of the 32Nd ACM SIGPLAN Conference on Programming

	Language Design and Implementation (PLDI ’11). ACM, New York,

	NY, USA, 175–186.

o	Susmit Sarkar, Kayvan Memarian, Scott Owens, Mark Batty,

	Peter Sewell, Luc Maranget, Jade Alglave, and Derek Williams.

	2012. "Synchronising C/C++ and POWER". In Proceedings of the 33rd

	ACM SIGPLAN Conference on Programming Language Design and

	Implementation (PLDI '12). ACM, New York, NY, USA, 311-322.

o	ARM Ltd. (Ed.). 2014. "ARM Architecture Reference Manual (ARMv8,

	for ARMv8-A architecture profile)". ARM Ltd.

o	Imagination Technologies, LTD. 2015. "MIPS(R) Architecture

	For Programmers, Volume II-A: The MIPS64(R) Instruction,

	Set Reference Manual". Imagination Technologies,

	LTD. https://imgtec.com/?do-download=4302.

o	Shaked Flur, Kathryn E. Gray, Christopher Pulte, Susmit

	Sarkar, Ali Sezgin, Luc Maranget, Will Deacon, and Peter

	Sewell. 2016. "Modelling the ARMv8 Architecture, Operationally:

	Concurrency and ISA". In Proceedings of the 43rd Annual ACM

	SIGPLAN-SIGACT Symposium on Principles of Programming Languages

	(POPL ’16). ACM, New York, NY, USA, 608–621.

o	Shaked Flur, Susmit Sarkar, Christopher Pulte, Kyndylan Nienhuis,

	Luc Maranget, Kathryn E. Gray, Ali Sezgin, Mark Batty, and Peter

	Sewell. 2017. "Mixed-size Concurrency: ARM, POWER, C/C++11,

	and SC". In Proceedings of the 44th ACM SIGPLAN Symposium on

	Principles of Programming Languages (POPL 2017). ACM, New York,

	NY, USA, 429–442.

Linux-kernel memory model

=========================

o	Andrea Parri, Alan Stern, Luc Maranget, Paul E. McKenney,

	and Jade Alglave.  2017. "A formal model of

	Linux-kernel memory ordering - companion webpage".

	http://moscova.inria.fr/∼maranget/cats7/linux/. (2017). [Online;

	accessed 30-January-2017].

o	Jade Alglave, Luc Maranget, Paul E. McKenney, Andrea Parri, and

	Alan Stern.  2017.  "A formal kernel memory-ordering model (part 1)"

	Linux Weekly News.  https://lwn.net/Articles/718628/

o	Jade Alglave, Luc Maranget, Paul E. McKenney, Andrea Parri, and

	Alan Stern.  2017.  "A formal kernel memory-ordering model (part 2)"

	Linux Weekly News.  https://lwn.net/Articles/720550/

Memory-model tooling

====================

o	Daniel Jackson. 2002. "Alloy: A Lightweight Object Modelling

	Notation". ACM Trans. Softw. Eng. Methodol. 11, 2 (April 2002),

	256–290. http://doi.acm.org/10.1145/505145.505149

o	Jade Alglave, Luc Maranget, and Michael Tautschnig. 2014. "Herding

	Cats: Modelling, Simulation, Testing, and Data Mining for Weak

	Memory". ACM Trans. Program. Lang. Syst. 36, 2, Article 7 (July

	2014), 7:1–7:74 pages.

o	Jade Alglave, Patrick Cousot, and Luc Maranget. 2016. "Syntax and

	semantics of the weak consistency model specification language

	cat". CoRR abs/1608.07531 (2016). http://arxiv.org/abs/1608.07531

Memory-model comparisons

========================

o	Paul E. McKenney, Ulrich Weigand, Andrea Parri, and Boqun

	Feng. 2016. "Linux-Kernel Memory Model". (6 June 2016).

	http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0124r2.html.