memory-barriers: Replace uses of "transitive"
The current version of memory-barriers.txt misuses the term "transitive", so this commit replaces it with multi-copy atomic, also adding a definition of this term. Reported-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Paul E. McKenney
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@ -53,7 +53,7 @@ CONTENTS
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- SMP barrier pairing.
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- Examples of memory barrier sequences.
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- Read memory barriers vs load speculation.
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- Transitivity
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- Multicopy atomicity.
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(*) Explicit kernel barriers.
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@ -635,6 +635,11 @@ can be used to record rare error conditions and the like, and the CPUs'
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naturally occurring ordering prevents such records from being lost.
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Note well that the ordering provided by a data dependency is local to
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the CPU containing it. See the section on "Multicopy atomicity" for
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more information.
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The data dependency barrier is very important to the RCU system,
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for example. See rcu_assign_pointer() and rcu_dereference() in
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include/linux/rcupdate.h. This permits the current target of an RCU'd
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@ -851,38 +856,11 @@ In short, control dependencies apply only to the stores in the then-clause
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and else-clause of the if-statement in question (including functions
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invoked by those two clauses), not to code following that if-statement.
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Finally, control dependencies do -not- provide transitivity. This is
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demonstrated by two related examples, with the initial values of
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'x' and 'y' both being zero:
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CPU 0 CPU 1
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======================= =======================
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r1 = READ_ONCE(x); r2 = READ_ONCE(y);
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if (r1 > 0) if (r2 > 0)
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WRITE_ONCE(y, 1); WRITE_ONCE(x, 1);
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Note well that the ordering provided by a control dependency is local
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to the CPU containing it. See the section on "Multicopy atomicity"
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for more information.
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assert(!(r1 == 1 && r2 == 1));
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The above two-CPU example will never trigger the assert(). However,
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if control dependencies guaranteed transitivity (which they do not),
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then adding the following CPU would guarantee a related assertion:
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CPU 2
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=====================
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WRITE_ONCE(x, 2);
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assert(!(r1 == 2 && r2 == 1 && x == 2)); /* FAILS!!! */
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But because control dependencies do -not- provide transitivity, the above
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assertion can fail after the combined three-CPU example completes. If you
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need the three-CPU example to provide ordering, you will need smp_mb()
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between the loads and stores in the CPU 0 and CPU 1 code fragments,
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that is, just before or just after the "if" statements. Furthermore,
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the original two-CPU example is very fragile and should be avoided.
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These two examples are the LB and WWC litmus tests from this paper:
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http://www.cl.cam.ac.uk/users/pes20/ppc-supplemental/test6.pdf and this
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site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html.
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In summary:
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@ -922,8 +900,8 @@ In summary:
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(*) Control dependencies pair normally with other types of barriers.
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(*) Control dependencies do -not- provide transitivity. If you
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need transitivity, use smp_mb().
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(*) Control dependencies do -not- provide multicopy atomicity. If you
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need all the CPUs to see a given store at the same time, use smp_mb().
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(*) Compilers do not understand control dependencies. It is therefore
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your job to ensure that they do not break your code.
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@ -936,13 +914,14 @@ When dealing with CPU-CPU interactions, certain types of memory barrier should
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always be paired. A lack of appropriate pairing is almost certainly an error.
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General barriers pair with each other, though they also pair with most
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other types of barriers, albeit without transitivity. An acquire barrier
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pairs with a release barrier, but both may also pair with other barriers,
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including of course general barriers. A write barrier pairs with a data
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dependency barrier, a control dependency, an acquire barrier, a release
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barrier, a read barrier, or a general barrier. Similarly a read barrier,
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control dependency, or a data dependency barrier pairs with a write
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barrier, an acquire barrier, a release barrier, or a general barrier:
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other types of barriers, albeit without multicopy atomicity. An acquire
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barrier pairs with a release barrier, but both may also pair with other
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barriers, including of course general barriers. A write barrier pairs
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with a data dependency barrier, a control dependency, an acquire barrier,
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a release barrier, a read barrier, or a general barrier. Similarly a
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read barrier, control dependency, or a data dependency barrier pairs
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with a write barrier, an acquire barrier, a release barrier, or a
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general barrier:
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CPU 1 CPU 2
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=============== ===============
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@ -1359,64 +1338,77 @@ the speculation will be cancelled and the value reloaded:
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retrieved : : +-------+
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TRANSITIVITY
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------------
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MULTICOPY ATOMICITY
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--------------------
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Transitivity is a deeply intuitive notion about ordering that is not
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always provided by real computer systems. The following example
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demonstrates transitivity:
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Multicopy atomicity is a deeply intuitive notion about ordering that is
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not always provided by real computer systems, namely that a given store
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is visible at the same time to all CPUs, or, alternatively, that all
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CPUs agree on the order in which all stores took place. However, use of
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full multicopy atomicity would rule out valuable hardware optimizations,
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so a weaker form called ``other multicopy atomicity'' instead guarantees
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that a given store is observed at the same time by all -other- CPUs. The
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remainder of this document discusses this weaker form, but for brevity
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will call it simply ``multicopy atomicity''.
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The following example demonstrates multicopy atomicity:
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CPU 1 CPU 2 CPU 3
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======================= ======================= =======================
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{ X = 0, Y = 0 }
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STORE X=1 LOAD X STORE Y=1
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<general barrier> <general barrier>
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LOAD Y LOAD X
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STORE X=1 r1=LOAD X (reads 1) LOAD Y (reads 1)
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<general barrier> <read barrier>
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STORE Y=r1 LOAD X
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Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
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This indicates that CPU 2's load from X in some sense follows CPU 1's
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store to X and that CPU 2's load from Y in some sense preceded CPU 3's
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store to Y. The question is then "Can CPU 3's load from X return 0?"
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Suppose that CPU 2's load from X returns 1 which it then stores to Y and
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that CPU 3's load from Y returns 1. This indicates that CPU 2's load
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from X in some sense follows CPU 1's store to X and that CPU 2's store
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to Y in some sense preceded CPU 3's load from Y. The question is then
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"Can CPU 3's load from X return 0?"
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Because CPU 2's load from X in some sense came after CPU 1's store, it
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Because CPU 3's load from X in some sense came after CPU 2's load, it
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is natural to expect that CPU 3's load from X must therefore return 1.
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This expectation is an example of transitivity: if a load executing on
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CPU A follows a load from the same variable executing on CPU B, then
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CPU A's load must either return the same value that CPU B's load did,
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or must return some later value.
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This expectation is an example of multicopy atomicity: if a load executing
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on CPU A follows a load from the same variable executing on CPU B, then
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an understandable but incorrect expectation is that CPU A's load must
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either return the same value that CPU B's load did, or must return some
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later value.
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In the Linux kernel, use of general memory barriers guarantees
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transitivity. Therefore, in the above example, if CPU 2's load from X
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returns 1 and its load from Y returns 0, then CPU 3's load from X must
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also return 1.
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In the Linux kernel, the above use of a general memory barrier compensates
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for any lack of multicopy atomicity. Therefore, in the above example,
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if CPU 2's load from X returns 1 and its load from Y returns 0, and CPU 3's
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load from Y returns 1, then CPU 3's load from X must also return 1.
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However, transitivity is -not- guaranteed for read or write barriers.
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For example, suppose that CPU 2's general barrier in the above example
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is changed to a read barrier as shown below:
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However, dependencies, read barriers, and write barriers are not always
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able to compensate for non-multicopy atomicity. For example, suppose
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that CPU 2's general barrier is removed from the above example, leaving
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only the data dependency shown below:
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CPU 1 CPU 2 CPU 3
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======================= ======================= =======================
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{ X = 0, Y = 0 }
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STORE X=1 LOAD X STORE Y=1
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<read barrier> <general barrier>
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LOAD Y LOAD X
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STORE X=1 r1=LOAD X (reads 1) LOAD Y (reads 1)
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<data dependency> <read barrier>
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STORE Y=r1 LOAD X (reads 0)
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This substitution destroys transitivity: in this example, it is perfectly
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legal for CPU 2's load from X to return 1, its load from Y to return 0,
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and CPU 3's load from X to return 0.
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This substitution allows non-multicopy atomicity to run rampant: in
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this example, it is perfectly legal for CPU 2's load from X to return 1,
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CPU 3's load from Y to return 1, and its load from X to return 0.
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The key point is that although CPU 2's read barrier orders its pair
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of loads, it does not guarantee to order CPU 1's store. Therefore, if
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this example runs on a system where CPUs 1 and 2 share a store buffer
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or a level of cache, CPU 2 might have early access to CPU 1's writes.
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General barriers are therefore required to ensure that all CPUs agree
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on the combined order of CPU 1's and CPU 2's accesses.
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The key point is that although CPU 2's data dependency orders its load
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and store, it does not guarantee to order CPU 1's store. Therefore,
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if this example runs on a non-multicopy-atomic system where CPUs 1 and 2
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share a store buffer or a level of cache, CPU 2 might have early access
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to CPU 1's writes. A general barrier is therefore required to ensure
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that all CPUs agree on the combined order of CPU 1's and CPU 2's accesses.
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General barriers provide "global transitivity", so that all CPUs will
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agree on the order of operations. In contrast, a chain of release-acquire
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pairs provides only "local transitivity", so that only those CPUs on
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the chain are guaranteed to agree on the combined order of the accesses.
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For example, switching to C code in deference to Herman Hollerith:
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General barriers can compensate not only for non-multicopy atomicity,
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but can also generate additional ordering that can ensure that -all-
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CPUs will perceive the same order of -all- operations. In contrast, a
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chain of release-acquire pairs do not provide this additional ordering,
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which means that only those CPUs on the chain are guaranteed to agree
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on the combined order of the accesses. For example, switching to C code
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in deference to the ghost of Herman Hollerith:
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int u, v, x, y, z;
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@ -1448,9 +1440,9 @@ For example, switching to C code in deference to Herman Hollerith:
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r3 = READ_ONCE(u);
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}
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Because cpu0(), cpu1(), and cpu2() participate in a local transitive
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chain of smp_store_release()/smp_load_acquire() pairs, the following
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outcome is prohibited:
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Because cpu0(), cpu1(), and cpu2() participate in a chain of
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smp_store_release()/smp_load_acquire() pairs, the following outcome
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is prohibited:
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r0 == 1 && r1 == 1 && r2 == 1
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@ -1460,9 +1452,9 @@ outcome is prohibited:
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r1 == 1 && r5 == 0
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However, the transitivity of release-acquire is local to the participating
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CPUs and does not apply to cpu3(). Therefore, the following outcome
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is possible:
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However, the ordering provided by a release-acquire chain is local
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to the CPUs participating in that chain and does not apply to cpu3(),
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at least aside from stores. Therefore, the following outcome is possible:
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r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
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@ -1490,8 +1482,8 @@ following outcome is possible:
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Note that this outcome can happen even on a mythical sequentially
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consistent system where nothing is ever reordered.
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To reiterate, if your code requires global transitivity, use general
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barriers throughout.
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To reiterate, if your code requires full ordering of all operations,
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use general barriers throughout.
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========================
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@ -3101,6 +3093,9 @@ AMD64 Architecture Programmer's Manual Volume 2: System Programming
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Chapter 7.1: Memory-Access Ordering
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Chapter 7.4: Buffering and Combining Memory Writes
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ARM Architecture Reference Manual (ARMv8, for ARMv8-A architecture profile)
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Chapter B2: The AArch64 Application Level Memory Model
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IA-32 Intel Architecture Software Developer's Manual, Volume 3:
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System Programming Guide
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Chapter 7.1: Locked Atomic Operations
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@ -3112,6 +3107,8 @@ The SPARC Architecture Manual, Version 9
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Appendix D: Formal Specification of the Memory Models
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Appendix J: Programming with the Memory Models
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Storage in the PowerPC (Stone and Fitzgerald)
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UltraSPARC Programmer Reference Manual
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Chapter 5: Memory Accesses and Cacheability
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Chapter 15: Sparc-V9 Memory Models
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