273 строки
9.0 KiB
Plaintext
273 строки
9.0 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 7.52 or higher of the "herd7" and "klitmus7" tools must be
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downloaded 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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Note that although these tools usually provide backwards compatibility,
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this is not absolutely guaranteed. Therefore, if a later version does
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not work, please try using the exact version called out above.
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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+fencembonceonces.litmus against the memory model:
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$ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
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Here is the corresponding output:
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Test SB+fencembonceonces 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+fencembonceonces Never 0 3
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Time SB+fencembonceonces 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+fencembonceonces.litmus against hardware:
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$ mkdir mymodules
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$ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.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+fencembonceonces 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+fencembonceonces Never 0 2000000
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Time SB+fencembonceonces 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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scripts Various scripts, see scripts/README.
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===========
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LIMITATIONS
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===========
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The Linux-kernel memory model (LKMM) has the following limitations:
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1. Compiler optimizations are not accurately modeled. Of course,
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the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
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ability to optimize, but under some circumstances it is possible
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for the compiler to undermine the memory model. For more
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information, see Documentation/explanation.txt (in particular,
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the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING"
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sections).
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Note that this limitation in turn limits LKMM's ability to
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accurately model address, control, and data dependencies.
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For example, if the compiler can deduce the value of some variable
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carrying a dependency, then the compiler can break that dependency
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by substituting a constant of that value.
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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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a. When rcu_assign_pointer() is passed NULL, the Linux
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kernel provides no ordering, but LKMM models this
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case as a store release.
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b. The "unless" RMW operations are not currently modeled:
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atomic_long_add_unless(), atomic_add_unless(),
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atomic_inc_unless_negative(), and
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atomic_dec_unless_positive(). These can be emulated
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in litmus tests, for example, by using atomic_cmpxchg().
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c. The call_rcu() function is not modeled. It can be
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emulated in litmus tests by adding another process that
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invokes synchronize_rcu() and the body of the callback
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function, with (for example) a release-acquire from
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the site of the emulated call_rcu() to the beginning
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of the additional process.
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d. The rcu_barrier() function is not modeled. It can be
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emulated in litmus tests emulating call_rcu() via
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(for example) a release-acquire from the end of each
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additional call_rcu() process to the site of the
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emulated rcu-barrier().
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e. Although sleepable RCU (SRCU) is now modeled, there
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are some subtle differences between its semantics and
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those in the Linux kernel. For example, the kernel
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might interpret the following sequence as two partially
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overlapping SRCU read-side critical sections:
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1 r1 = srcu_read_lock(&my_srcu);
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2 do_something_1();
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3 r2 = srcu_read_lock(&my_srcu);
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4 do_something_2();
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5 srcu_read_unlock(&my_srcu, r1);
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6 do_something_3();
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7 srcu_read_unlock(&my_srcu, r2);
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In contrast, LKMM will interpret this as a nested pair of
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SRCU read-side critical sections, with the outer critical
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section spanning lines 1-7 and the inner critical section
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spanning lines 3-5.
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This difference would be more of a concern had anyone
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identified a reasonable use case for partially overlapping
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SRCU read-side critical sections. For more information,
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please see: https://paulmck.livejournal.com/40593.html
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f. Reader-writer locking is not modeled. It can be
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emulated in litmus tests using atomic read-modify-write
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operations.
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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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Finally, please note that LKMM is subject to change as hardware, use cases,
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and compilers evolve.
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