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Herbert Xu recently reported a problem concerning RCU and compiler barriers. In the course of discussing the problem, he put forth a litmus test which illustrated a serious defect in the Linux Kernel Memory Model's data-race-detection code [1]. The defect was that the LKMM assumed visibility and executes-before ordering of plain accesses had to be mediated by marked accesses. In Herbert's litmus test this wasn't so, and the LKMM claimed the litmus test was allowed and contained a data race although neither is true. In fact, plain accesses can be ordered by fences even in the absence of marked accesses. In most cases this doesn't matter, because most fences only order accesses within a single thread. But the rcu-fence relation is different; it can order (and induce visibility between) accesses in different threads -- events which otherwise might be concurrent. This makes it relevant to data-race detection. This patch makes two changes to the memory model to incorporate the new insight: If a store is separated by a fence from another access, the store is necessarily visible to the other access (as reflected in the ww-vis and wr-vis relations). Similarly, if a load is separated by a fence from another access then the load necessarily executes before the other access (as reflected in the rw-xbstar relation). If a store is separated by a strong fence from a marked access then it is necessarily visible to any access that executes after the marked access (as reflected in the ww-vis and wr-vis relations). With these changes, the LKMM gives the desired result for Herbert's litmus test and other related ones [2]. [1] https://lore.kernel.org/lkml/Pine.LNX.4.44L0.1906041026570.1731-100000@iolanthe.rowland.org/ [2] https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-1.litmus https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-2.litmus https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-3.litmus https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-4.litmus https://github.com/paulmckrcu/litmus/blob/master/manual/plain/strong-vis.litmus Reported-by: Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Tested-by: Akira Yokosawa <akiyks@gmail.com> |
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| Documentation | ||
| litmus-tests | ||
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| linux-kernel.bell | ||
| linux-kernel.cat | ||
| linux-kernel.cfg | ||
| linux-kernel.def | ||
| lock.cat | ||
| README | ||
===================================== 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 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). 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_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. 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.