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Because the docker and gentoo images haven't been updated in quite some time, they are likely to provide more confusion than help. This commit therefore removes mention of them from the README file. Reported-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: akiyks@gmail.com Cc: boqun.feng@gmail.com Cc: dhowells@redhat.com Cc: j.alglave@ucl.ac.uk Cc: linux-arch@vger.kernel.org Cc: luc.maranget@inria.fr Cc: npiggin@gmail.com Cc: parri.andrea@gmail.com Cc: will.deacon@arm.com Link: http://lkml.kernel.org/r/1520443660-16858-2-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
207 lines
6.3 KiB
Plaintext
207 lines
6.3 KiB
Plaintext
=====================================
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LINUX KERNEL MEMORY CONSISTENCY MODEL
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=====================================
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============
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INTRODUCTION
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============
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This directory contains the memory consistency model (memory model, for
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short) of the Linux kernel, written in the "cat" language and executable
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by the externally provided "herd7" simulator, which exhaustively explores
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the state space of small litmus tests.
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In addition, the "klitmus7" tool (also externally provided) may be used
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to convert a litmus test to a Linux kernel module, which in turn allows
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that litmus test to be exercised within the Linux kernel.
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============
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REQUIREMENTS
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============
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Version 7.48 of the "herd7" and "klitmus7" tools must be downloaded
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separately:
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https://github.com/herd/herdtools7
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See "herdtools7/INSTALL.md" for installation instructions.
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==================
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BASIC USAGE: HERD7
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==================
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The memory model is used, in conjunction with "herd7", to exhaustively
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explore the state space of small litmus tests.
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For example, to run SB+mbonceonces.litmus against the memory model:
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$ herd7 -conf linux-kernel.cfg litmus-tests/SB+mbonceonces.litmus
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Here is the corresponding output:
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Test SB+mbonceonces Allowed
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States 3
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0:r0=0; 1:r0=1;
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0:r0=1; 1:r0=0;
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0:r0=1; 1:r0=1;
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No
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Witnesses
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Positive: 0 Negative: 3
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Condition exists (0:r0=0 /\ 1:r0=0)
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Observation SB+mbonceonces Never 0 3
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Time SB+mbonceonces 0.01
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Hash=d66d99523e2cac6b06e66f4c995ebb48
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The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
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this litmus test's "exists" clause can not be satisfied.
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See "herd7 -help" or "herdtools7/doc/" for more information.
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=====================
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BASIC USAGE: KLITMUS7
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=====================
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The "klitmus7" tool converts a litmus test into a Linux kernel module,
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which may then be loaded and run.
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For example, to run SB+mbonceonces.litmus against hardware:
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$ mkdir mymodules
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$ klitmus7 -o mymodules litmus-tests/SB+mbonceonces.litmus
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$ cd mymodules ; make
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$ sudo sh run.sh
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The corresponding output includes:
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Test SB+mbonceonces Allowed
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Histogram (3 states)
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644580 :>0:r0=1; 1:r0=0;
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644328 :>0:r0=0; 1:r0=1;
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711092 :>0:r0=1; 1:r0=1;
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No
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Witnesses
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Positive: 0, Negative: 2000000
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Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
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Hash=d66d99523e2cac6b06e66f4c995ebb48
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Observation SB+mbonceonces Never 0 2000000
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Time SB+mbonceonces 0.16
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The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
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that during two million trials, the state specified in this litmus
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test's "exists" clause was not reached.
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And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
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for more information.
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====================
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DESCRIPTION OF FILES
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====================
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Documentation/cheatsheet.txt
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Quick-reference guide to the Linux-kernel memory model.
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Documentation/explanation.txt
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Describes the memory model in detail.
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Documentation/recipes.txt
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Lists common memory-ordering patterns.
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Documentation/references.txt
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Provides background reading.
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linux-kernel.bell
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Categorizes the relevant instructions, including memory
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references, memory barriers, atomic read-modify-write operations,
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lock acquisition/release, and RCU operations.
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More formally, this file (1) lists the subtypes of the various
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event types used by the memory model and (2) performs RCU
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read-side critical section nesting analysis.
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linux-kernel.cat
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Specifies what reorderings are forbidden by memory references,
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memory barriers, atomic read-modify-write operations, and RCU.
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More formally, this file specifies what executions are forbidden
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by the memory model. Allowed executions are those which
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satisfy the model's "coherence", "atomic", "happens-before",
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"propagation", and "rcu" axioms, which are defined in the file.
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linux-kernel.cfg
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Convenience file that gathers the common-case herd7 command-line
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arguments.
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linux-kernel.def
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Maps from C-like syntax to herd7's internal litmus-test
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instruction-set architecture.
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litmus-tests
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Directory containing a few representative litmus tests, which
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are listed in litmus-tests/README. A great deal more litmus
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tests are available at https://github.com/paulmckrcu/litmus.
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lock.cat
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Provides a front-end analysis of lock acquisition and release,
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for example, associating a lock acquisition with the preceding
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and following releases and checking for self-deadlock.
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More formally, this file defines a performance-enhanced scheme
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for generation of the possible reads-from and coherence order
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relations on the locking primitives.
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README
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This file.
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===========
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LIMITATIONS
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===========
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The Linux-kernel memory model has the following limitations:
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1. Compiler optimizations are not modeled. Of course, the use
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of READ_ONCE() and WRITE_ONCE() limits the compiler's ability
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to optimize, but there is Linux-kernel code that uses bare C
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memory accesses. Handling this code is on the to-do list.
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For more information, see Documentation/explanation.txt (in
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particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"
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and "A WARNING" sections).
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2. Multiple access sizes for a single variable are not supported,
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and neither are misaligned or partially overlapping accesses.
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3. Exceptions and interrupts are not modeled. In some cases,
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this limitation can be overcome by modeling the interrupt or
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exception with an additional process.
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4. I/O such as MMIO or DMA is not supported.
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5. Self-modifying code (such as that found in the kernel's
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alternatives mechanism, function tracer, Berkeley Packet Filter
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JIT compiler, and module loader) is not supported.
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6. Complete modeling of all variants of atomic read-modify-write
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operations, locking primitives, and RCU is not provided.
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For example, call_rcu() and rcu_barrier() are not supported.
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However, a substantial amount of support is provided for these
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operations, as shown in the linux-kernel.def file.
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The "herd7" tool has some additional limitations of its own, apart from
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the memory model:
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1. Non-trivial data structures such as arrays or structures are
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not supported. However, pointers are supported, allowing trivial
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linked lists to be constructed.
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2. Dynamic memory allocation is not supported, although this can
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be worked around in some cases by supplying multiple statically
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allocated variables.
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Some of these limitations may be overcome in the future, but others are
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more likely to be addressed by incorporating the Linux-kernel memory model
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into other tools.
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