Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+		=====================================
+		LINUX KERNEL MEMORY CONSISTENCY MODEL
+		=====================================
+
+============
+INTRODUCTION
+============
+
+This directory contains the memory consistency model (memory model, for
+short) of the Linux kernel, written in the "cat" language and executable
+by the externally provided "herd7" simulator, which exhaustively explores
+the state space of small litmus tests.
+
+In addition, the "klitmus7" tool (also externally provided) may be used
+to convert a litmus test to a Linux kernel module, which in turn allows
+that litmus test to be exercised within the Linux kernel.
+
+
+============
+REQUIREMENTS
+============
+
+Version 7.49 of the "herd7" and "klitmus7" tools must be downloaded
+separately:
+
+  https://github.com/herd/herdtools7
+
+See "herdtools7/INSTALL.md" for installation instructions.
+
+
+==================
+BASIC USAGE: HERD7
+==================
+
+The memory model is used, in conjunction with "herd7", to exhaustively
+explore the state space of small litmus tests.
+
+For example, to run SB+fencembonceonces.litmus against the memory model:
+
+  $ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
+
+Here is the corresponding output:
+
+  Test SB+fencembonceonces Allowed
+  States 3
+  0:r0=0; 1:r0=1;
+  0:r0=1; 1:r0=0;
+  0:r0=1; 1:r0=1;
+  No
+  Witnesses
+  Positive: 0 Negative: 3
+  Condition exists (0:r0=0 /\ 1:r0=0)
+  Observation SB+fencembonceonces Never 0 3
+  Time SB+fencembonceonces 0.01
+  Hash=d66d99523e2cac6b06e66f4c995ebb48
+
+The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
+this litmus test's "exists" clause can not be satisfied.
+
+See "herd7 -help" or "herdtools7/doc/" for more information.
+
+
+=====================
+BASIC USAGE: KLITMUS7
+=====================
+
+The "klitmus7" tool converts a litmus test into a Linux kernel module,
+which may then be loaded and run.
+
+For example, to run SB+fencembonceonces.litmus against hardware:
+
+  $ mkdir mymodules
+  $ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus
+  $ cd mymodules ; make
+  $ sudo sh run.sh
+
+The corresponding output includes:
+
+  Test SB+fencembonceonces Allowed
+  Histogram (3 states)
+  644580  :>0:r0=1; 1:r0=0;
+  644328  :>0:r0=0; 1:r0=1;
+  711092  :>0:r0=1; 1:r0=1;
+  No
+  Witnesses
+  Positive: 0, Negative: 2000000
+  Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
+  Hash=d66d99523e2cac6b06e66f4c995ebb48
+  Observation SB+fencembonceonces Never 0 2000000
+  Time SB+fencembonceonces 0.16
+
+The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
+that during two million trials, the state specified in this litmus
+test's "exists" clause was not reached.
+
+And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
+for more information.
+
+
+====================
+DESCRIPTION OF FILES
+====================
+
+Documentation/cheatsheet.txt
+	Quick-reference guide to the Linux-kernel memory model.
+
+Documentation/explanation.txt
+	Describes the memory model in detail.
+
+Documentation/recipes.txt
+	Lists common memory-ordering patterns.
+
+Documentation/references.txt
+	Provides background reading.
+
+linux-kernel.bell
+	Categorizes the relevant instructions, including memory
+	references, memory barriers, atomic read-modify-write operations,
+	lock acquisition/release, and RCU operations.
+
+	More formally, this file (1) lists the subtypes of the various
+	event types used by the memory model and (2) performs RCU
+	read-side critical section nesting analysis.
+
+linux-kernel.cat
+	Specifies what reorderings are forbidden by memory references,
+	memory barriers, atomic read-modify-write operations, and RCU.
+
+	More formally, this file specifies what executions are forbidden
+	by the memory model.  Allowed executions are those which
+	satisfy the model's "coherence", "atomic", "happens-before",
+	"propagation", and "rcu" axioms, which are defined in the file.
+
+linux-kernel.cfg
+	Convenience file that gathers the common-case herd7 command-line
+	arguments.
+
+linux-kernel.def
+	Maps from C-like syntax to herd7's internal litmus-test
+	instruction-set architecture.
+
+litmus-tests
+	Directory containing a few representative litmus tests, which
+	are listed in litmus-tests/README.  A great deal more litmus
+	tests are available at https://github.com/paulmckrcu/litmus.
+
+lock.cat
+	Provides a front-end analysis of lock acquisition and release,
+	for example, associating a lock acquisition with the preceding
+	and following releases and checking for self-deadlock.
+
+	More formally, this file defines a performance-enhanced scheme
+	for generation of the possible reads-from and coherence order
+	relations on the locking primitives.
+
+README
+	This file.
+
+
+===========
+LIMITATIONS
+===========
+
+The Linux-kernel memory model has the following limitations:
+
+1.	Compiler optimizations are not modeled.  Of course, the use
+	of READ_ONCE() and WRITE_ONCE() limits the compiler's ability
+	to optimize, but there is Linux-kernel code that uses bare C
+	memory accesses.  Handling this code is on the to-do list.
+	For more information, see Documentation/explanation.txt (in
+	particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"
+	and "A WARNING" sections).
+
+2.	Multiple access sizes for a single variable are not supported,
+	and neither are misaligned or partially overlapping accesses.
+
+3.	Exceptions and interrupts are not modeled.  In some cases,
+	this limitation can be overcome by modeling the interrupt or
+	exception with an additional process.
+
+4.	I/O such as MMIO or DMA is not supported.
+
+5.	Self-modifying code (such as that found in the kernel's
+	alternatives mechanism, function tracer, Berkeley Packet Filter
+	JIT compiler, and module loader) is not supported.
+
+6.	Complete modeling of all variants of atomic read-modify-write
+	operations, locking primitives, and RCU is not provided.
+	For example, call_rcu() and rcu_barrier() are not supported.
+	However, a substantial amount of support is provided for these
+	operations, as shown in the linux-kernel.def file.
+
+The "herd7" tool has some additional limitations of its own, apart from
+the memory model:
+
+1.	Non-trivial data structures such as arrays or structures are
+	not supported.	However, pointers are supported, allowing trivial
+	linked lists to be constructed.
+
+2.	Dynamic memory allocation is not supported, although this can
+	be worked around in some cases by supplying multiple statically
+	allocated variables.
+
+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.