diff options
Diffstat (limited to 'tools/memory-model/Documentation')
| -rw-r--r-- | tools/memory-model/Documentation/README | 7 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/explanation.txt | 2 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/locking.txt | 5 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/ordering.txt | 22 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/recipes.txt | 4 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/references.txt | 3 | ||||
| -rw-r--r-- | tools/memory-model/Documentation/simple.txt | 4 |
7 files changed, 29 insertions, 18 deletions
diff --git a/tools/memory-model/Documentation/README b/tools/memory-model/Documentation/README index 9999c1effdb6..88870b0bceea 100644 --- a/tools/memory-model/Documentation/README +++ b/tools/memory-model/Documentation/README @@ -23,8 +23,11 @@ o You are familiar with the Linux-kernel concurrency primitives that you need, and just want to get started with LKMM litmus tests: litmus-tests.txt -o You would like to access lock-protected shared variables without - having their corresponding locks held: locking.txt +o You need to locklessly access shared variables that are otherwise + protected by a lock: locking.txt + + This locking.txt file expands on the "Locking" section in + recipes.txt, but is self-contained. o You are familiar with Linux-kernel concurrency, and would like a detailed intuitive understanding of LKMM, including diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt index 6dc8b3642458..34aa3172071b 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -1896,7 +1896,7 @@ following respects: 3. The srcu_down_read() and srcu_up_read() primitives work exactly like srcu_read_lock() and srcu_read_unlock(), except - that matching calls don't have to execute on the same CPU. + that matching calls don't have to execute within the same context. (The names are meant to be suggestive of operations on semaphores.) Since the matching is determined by the domain pointer and index value, these primitives make it possible for diff --git a/tools/memory-model/Documentation/locking.txt b/tools/memory-model/Documentation/locking.txt index 65c898c64a93..d6dc3cc34ab6 100644 --- a/tools/memory-model/Documentation/locking.txt +++ b/tools/memory-model/Documentation/locking.txt @@ -1,3 +1,8 @@ +[!] Note: + This file expands on the "Locking" section of recipes.txt, + focusing on locklessly accessing shared variables that are + otherwise protected by a lock. + Locking ======= diff --git a/tools/memory-model/Documentation/ordering.txt b/tools/memory-model/Documentation/ordering.txt index 9b0949d3f5ec..7ab3744929d8 100644 --- a/tools/memory-model/Documentation/ordering.txt +++ b/tools/memory-model/Documentation/ordering.txt @@ -223,7 +223,7 @@ The Linux kernel's compiler barrier is barrier(). This primitive prohibits compiler code-motion optimizations that might move memory references across the point in the code containing the barrier(), but does not constrain hardware memory ordering. For example, this can be -used to prevent to compiler from moving code across an infinite loop: +used to prevent the compiler from moving code across an infinite loop: WRITE_ONCE(x, 1); while (dontstop) @@ -274,7 +274,7 @@ different pieces of the concurrent algorithm. The variable stored to by the smp_store_release(), in this case "y", will normally be used in an acquire operation in other parts of the concurrent algorithm. -To see the performance advantages, suppose that the above example read +To see the performance advantages, suppose that the above example reads from "x" instead of writing to it. Then an smp_wmb() could not guarantee ordering, and an smp_mb() would be needed instead: @@ -394,17 +394,17 @@ from the value returned by the rcu_dereference() or srcu_dereference() to that subsequent memory access. A call to rcu_dereference() for a given RCU-protected pointer is -usually paired with a call to a call to rcu_assign_pointer() for that -same pointer in much the same way that a call to smp_load_acquire() is -paired with a call to smp_store_release(). Calls to rcu_dereference() -and rcu_assign_pointer are often buried in other APIs, for example, +usually paired with a call to rcu_assign_pointer() for that same pointer +in much the same way that a call to smp_load_acquire() is paired with +a call to smp_store_release(). Calls to rcu_dereference() and +rcu_assign_pointer() are often buried in other APIs, for example, the RCU list API members defined in include/linux/rculist.h. For more information, please see the docbook headers in that file, the most -recent LWN article on the RCU API (https://lwn.net/Articles/777036/), +recent LWN article on the RCU API (https://lwn.net/Articles/988638/), and of course the material in Documentation/RCU. If the pointer value is manipulated between the rcu_dereference() -that returned it and a later dereference(), please read +that returned it and a later rcu_dereference(), please read Documentation/RCU/rcu_dereference.rst. It can also be quite helpful to review uses in the Linux kernel. @@ -457,7 +457,7 @@ described earlier in this document. These operations come in three categories: o Marked writes, such as WRITE_ONCE() and atomic_set(). These - primitives required the compiler to emit the corresponding store + primitives require the compiler to emit the corresponding store instructions in the expected execution order, thus suppressing a number of destructive optimizations. However, they provide no hardware ordering guarantees, and in fact many CPUs will happily @@ -465,7 +465,7 @@ o Marked writes, such as WRITE_ONCE() and atomic_set(). These operations, unless these operations are to the same variable. o Marked reads, such as READ_ONCE() and atomic_read(). These - primitives required the compiler to emit the corresponding load + primitives require the compiler to emit the corresponding load instructions in the expected execution order, thus suppressing a number of destructive optimizations. However, they provide no hardware ordering guarantees, and in fact many CPUs will happily @@ -506,7 +506,7 @@ of the old value and the new value. Unmarked C-language accesses are unordered, and are also subject to any number of compiler optimizations, many of which can break your -concurrent code. It is possible to used unmarked C-language accesses for +concurrent code. It is possible to use unmarked C-language accesses for shared variables that are subject to concurrent access, but great care is required on an ongoing basis. The compiler-constraining barrier() primitive can be helpful, as can the various ordering primitives discussed diff --git a/tools/memory-model/Documentation/recipes.txt b/tools/memory-model/Documentation/recipes.txt index 03f58b11c252..52115ee5f393 100644 --- a/tools/memory-model/Documentation/recipes.txt +++ b/tools/memory-model/Documentation/recipes.txt @@ -61,6 +61,10 @@ usual) some things to be careful of: Locking ------- +[!] Note: + locking.txt expands on this section, providing more detail on + locklessly accessing lock-protected shared variables. + Locking is well-known and straightforward, at least if you don't think about it too hard. And the basic rule is indeed quite simple: Any CPU that has acquired a given lock sees any changes previously seen or made by any diff --git a/tools/memory-model/Documentation/references.txt b/tools/memory-model/Documentation/references.txt index c5fdfd19df24..d691390620b3 100644 --- a/tools/memory-model/Documentation/references.txt +++ b/tools/memory-model/Documentation/references.txt @@ -46,8 +46,7 @@ o ARM Ltd. (Ed.). 2014. "ARM Architecture Reference Manual (ARMv8, o Imagination Technologies, LTD. 2015. "MIPS(R) Architecture For Programmers, Volume II-A: The MIPS64(R) Instruction, - Set Reference Manual". Imagination Technologies, - LTD. https://imgtec.com/?do-download=4302. + Set Reference Manual". Imagination Technologies, LTD. o Shaked Flur, Kathryn E. Gray, Christopher Pulte, Susmit Sarkar, Ali Sezgin, Luc Maranget, Will Deacon, and Peter diff --git a/tools/memory-model/Documentation/simple.txt b/tools/memory-model/Documentation/simple.txt index 21f06c1d1b70..2df148630cdc 100644 --- a/tools/memory-model/Documentation/simple.txt +++ b/tools/memory-model/Documentation/simple.txt @@ -134,7 +134,7 @@ Packaged primitives: Sequence locking Lockless programming is considered by many to be more difficult than lock-based programming, but there are a few lockless design patterns that have been built out into an API. One of these APIs is sequence locking. -Although this APIs can be used in extremely complex ways, there are simple +Although this API can be used in extremely complex ways, there are simple and effective ways of using it that avoid the need to pay attention to memory ordering. @@ -205,7 +205,7 @@ If you want to keep things simple, use the initialization and read-out operations from the previous section only when there are no racing accesses. Otherwise, use only fully ordered operations when accessing or modifying the variable. This approach guarantees that code prior -to a given access to that variable will be seen by all CPUs has having +to a given access to that variable will be seen by all CPUs as having happened before any code following any later access to that same variable. Please note that per-CPU functions are not atomic operations and |