tools/memory-model: Add more LKMM limitations

	particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"

	and "A WARNING" sections).

	Note that this limitation in turn limits LKMM's ability to

	accurately model address, control, and data dependencies.

	For example, if the compiler can deduce the value of some variable

	carrying a dependency, then the compiler can break that dependency

	by substituting a constant of that value.

2.	Multiple access sizes for a single variable are not supported,

	and neither are misaligned or partially overlapping accesses.

	However, a substantial amount of support is provided for these

	operations, as shown in the linux-kernel.def file.

	a.	When rcu_assign_pointer() is passed NULL, the Linux

		kernel provides no ordering, but LKMM models this

		case as a store release.

	b.	The "unless" RMW operations are not currently modeled:

		atomic_long_add_unless(), atomic_add_unless(),

		atomic_inc_unless_negative(), and

		atomic_dec_unless_positive().  These can be emulated

		in litmus tests, for example, by using atomic_cmpxchg().

	c.	The call_rcu() function is not modeled.  It can be

		emulated in litmus tests by adding another process that

		invokes synchronize_rcu() and the body of the callback

		function, with (for example) a release-acquire from

		the site of the emulated call_rcu() to the beginning

		of the additional process.

	d.	The rcu_barrier() function is not modeled.  It can be

		emulated in litmus tests emulating call_rcu() via

		(for example) a release-acquire from the end of each

		additional call_rcu() process to the site of the

		emulated rcu-barrier().

	e.	Sleepable RCU (SRCU) is not modeled.  It can be

		emulated, but perhaps not simply.

	f.	Reader-writer locking is not modeled.  It can be

		emulated in litmus tests using atomic read-modify-write

		operations.

The "herd7" tool has some additional limitations of its own, apart from

the memory model:

Some of these limitations may be overcome in the future, but others are

more likely to be addressed by incorporating the Linux-kernel memory model

into other tools.

Finally, please note that LKMM is subject to change as hardware, use cases,

and compilers evolve.