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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 7.52 or higher of the "herd7" and "klitmus7" tools must be
downloaded separately:
https://github.com/herd/herdtools7
See "herdtools7/INSTALL.md" for installation instructions.
Note that although these tools usually provide backwards compatibility,
this is not absolutely guaranteed. Therefore, if a later version does
not work, please try using the exact version called out above.
==================
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.
scripts Various scripts, see scripts/README.
===========
LIMITATIONS
===========
The Linux-kernel memory model (LKMM) has the following limitations:
1. Compiler optimizations are not accurately modeled. Of course,
the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
ability to optimize, but under some circumstances it is possible
for the compiler to undermine the memory model. For more
information, see Documentation/explanation.txt (in 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.
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.
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_inc_unless_negative(),
and atomic_dec_unless_positive(). These can be emulated
in litmus tests, for example, by using atomic_cmpxchg().
One exception of this limitation is atomic_add_unless(),
which is provided directly by herd7 (so no corresponding
definition in linux-kernel.def). atomic_add_unless() is
modeled by herd7 therefore it can be used in litmus tests.
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. Although sleepable RCU (SRCU) is now modeled, there
are some subtle differences between its semantics and
those in the Linux kernel. For example, the kernel
might interpret the following sequence as two partially
overlapping SRCU read-side critical sections:
1 r1 = srcu_read_lock(&my_srcu);
2 do_something_1();
3 r2 = srcu_read_lock(&my_srcu);
4 do_something_2();
5 srcu_read_unlock(&my_srcu, r1);
6 do_something_3();
7 srcu_read_unlock(&my_srcu, r2);
In contrast, LKMM will interpret this as a nested pair of
SRCU read-side critical sections, with the outer critical
section spanning lines 1-7 and the inner critical section
spanning lines 3-5.
This difference would be more of a concern had anyone
identified a reasonable use case for partially overlapping
SRCU read-side critical sections. For more information,
please see: https://paulmck.livejournal.com/40593.html
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:
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.
Finally, please note that LKMM is subject to change as hardware, use cases,
and compilers evolve.
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