Merge branch 'for-mingo' of...

     with memory references that are not protected by READ_ONCE() and

     WRITE_ONCE().  Without them, the compiler is within its rights to

     do all sorts of "creative" transformations, which are covered in

     the Compiler Barrier section.

     the COMPILER BARRIER section.

 (*) It _must_not_ be assumed that independent loads and stores will be issued

     in the order given.  This means that for:

This enforces the occurrence of one of the two implications, and prevents the

third possibility from arising.

A data-dependency barrier must also order against dependent writes:

	CPU 1		      CPU 2

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

	{ A == 1, B == 2, C = 3, P == &A, Q == &C }

	B = 4;

	<write barrier>

	WRITE_ONCE(P, &B);

			      Q = READ_ONCE(P);

			      <data dependency barrier>

			      *Q = 5;

The data-dependency barrier must order the read into Q with the store

into *Q.  This prohibits this outcome:

	(Q == B) && (B == 4)

Please note that this pattern should be rare.  After all, the whole point

of dependency ordering is to -prevent- writes to the data structure, along

with the expensive cache misses associated with those writes.  This pattern

can be used to record rare error conditions and the like, and the ordering

prevents such records from being lost.

[!] Note that this extremely counterintuitive situation arises most easily on

machines with split caches, so that, for example, one cache bank processes

even-numbered cache lines and the other bank processes odd-numbered cache

but the old value of the variable B (2).

Another example of where data dependency barriers might be required is where a

number is read from memory and then used to calculate the index for an array

access:

	CPU 1		      CPU 2

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

	{ M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }

	M[1] = 4;

	<write barrier>

	WRITE_ONCE(P, 1);

			      Q = READ_ONCE(P);

			      <data dependency barrier>

			      D = M[Q];

The data dependency barrier is very important to the RCU system,

for example.  See rcu_assign_pointer() and rcu_dereference() in

include/linux/rcupdate.h.  This permits the current target of an RCU'd

      use smp_rmb(), smp_wmb(), or, in the case of prior stores and

      later loads, smp_mb().

  (*) If both legs of the "if" statement begin with identical stores

      to the same variable, a barrier() statement is required at the

      beginning of each leg of the "if" statement.

  (*) If both legs of the "if" statement begin with identical stores to

      the same variable, then those stores must be ordered, either by

      preceding both of them with smp_mb() or by using smp_store_release()

      to carry out the stores.  Please note that it is -not- sufficient

      to use barrier() at beginning of each leg of the "if" statement,

      as optimizing compilers do not necessarily respect barrier()

      in this case.

  (*) Control dependencies require at least one run-time conditional

      between the prior load and the subsequent store, and this

  (*) Control dependencies require that the compiler avoid reordering the

      dependency into nonexistence.  Careful use of READ_ONCE() or

      atomic{,64}_read() can help to preserve your control dependency.

      Please see the Compiler Barrier section for more information.

      Please see the COMPILER BARRIER section for more information.

  (*) Control dependencies pair normally with other types of barriers.

Transitivity is a deeply intuitive notion about ordering that is not

always provided by real computer systems.  The following example

demonstrates transitivity (also called "cumulativity"):

demonstrates transitivity:

	CPU 1			CPU 2			CPU 3

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

General barriers are therefore required to ensure that all CPUs agree

on the combined order of CPU 1's and CPU 2's accesses.

To reiterate, if your code requires transitivity, use general barriers

throughout.

General barriers provide "global transitivity", so that all CPUs will

agree on the order of operations.  In contrast, a chain of release-acquire

pairs provides only "local transitivity", so that only those CPUs on

the chain are guaranteed to agree on the combined order of the accesses.

For example, switching to C code in deference to Herman Hollerith:

	int u, v, x, y, z;

	void cpu0(void)

	{

		r0 = smp_load_acquire(&x);

		WRITE_ONCE(u, 1);

		smp_store_release(&y, 1);

	}

	void cpu1(void)

	{

		r1 = smp_load_acquire(&y);

		r4 = READ_ONCE(v);

		r5 = READ_ONCE(u);

		smp_store_release(&z, 1);

	}

	void cpu2(void)

	{

		r2 = smp_load_acquire(&z);

		smp_store_release(&x, 1);

	}

	void cpu3(void)

	{

		WRITE_ONCE(v, 1);

		smp_mb();

		r3 = READ_ONCE(u);

	}

Because cpu0(), cpu1(), and cpu2() participate in a local transitive

chain of smp_store_release()/smp_load_acquire() pairs, the following

outcome is prohibited:

	r0 == 1 && r1 == 1 && r2 == 1

Furthermore, because of the release-acquire relationship between cpu0()

and cpu1(), cpu1() must see cpu0()'s writes, so that the following

outcome is prohibited:

	r1 == 1 && r5 == 0

However, the transitivity of release-acquire is local to the participating

CPUs and does not apply to cpu3().  Therefore, the following outcome

is possible:

	r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0

As an aside, the following outcome is also possible:

	r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1

Although cpu0(), cpu1(), and cpu2() will see their respective reads and

writes in order, CPUs not involved in the release-acquire chain might

well disagree on the order.  This disagreement stems from the fact that

the weak memory-barrier instructions used to implement smp_load_acquire()

and smp_store_release() are not required to order prior stores against

subsequent loads in all cases.  This means that cpu3() can see cpu0()'s

store to u as happening -after- cpu1()'s load from v, even though

both cpu0() and cpu1() agree that these two operations occurred in the

intended order.

However, please keep in mind that smp_load_acquire() is not magic.

In particular, it simply reads from its argument with ordering.  It does

-not- ensure that any particular value will be read.  Therefore, the

following outcome is possible:

	r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0

Note that this outcome can happen even on a mythical sequentially

consistent system where nothing is ever reordered.

To reiterate, if your code requires global transitivity, use general

barriers throughout.

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

     the following:

	a = 0;

	/* Code that does not store to variable a. */

	... Code that does not store to variable a ...

	a = 0;

     The compiler sees that the value of variable 'a' is already zero, so

     wrong guess:

	WRITE_ONCE(a, 0);

	/* Code that does not store to variable a. */

	... Code that does not store to variable a ...

	WRITE_ONCE(a, 0);

 (*) The compiler is within its rights to reorder memory accesses unless