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memory-barriers: Rework multicopy-atomicity section

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Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Alan Stern 2017-09-01 07:53:34 -07:00 committed by Paul E. McKenney
parent f1ab25a30c
commit 0902b1f44a
1 changed files with 29 additions and 27 deletions
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Documentation/memory-barriers.txt
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@ -1343,13 +1343,13 @@ MULTICOPY ATOMICITY
Multicopy atomicity is a deeply intuitive notion about ordering that is
not always provided by real computer systems, namely that a given store
is visible at the same time to all CPUs, or, alternatively, that all
CPUs agree on the order in which all stores took place. However, use of
full multicopy atomicity would rule out valuable hardware optimizations,
so a weaker form called ``other multicopy atomicity'' instead guarantees
that a given store is observed at the same time by all -other- CPUs. The
remainder of this document discusses this weaker form, but for brevity
will call it simply ``multicopy atomicity''.
becomes visible at the same time to all CPUs, or, alternatively, that all
CPUs agree on the order in which all stores become visible. However,
support of full multicopy atomicity would rule out valuable hardware
optimizations, so a weaker form called ``other multicopy atomicity''
instead guarantees only that a given store becomes visible at the same
time to all -other- CPUs. The remainder of this document discusses this
weaker form, but for brevity will call it simply ``multicopy atomicity''.
The following example demonstrates multicopy atomicity:
@ -1360,24 +1360,26 @@ The following example demonstrates multicopy atomicity:
<general barrier> <read barrier>
STORE Y=r1 LOAD X
Suppose that CPU 2's load from X returns 1 which it then stores to Y and
that CPU 3's load from Y returns 1. This indicates that CPU 2's load
from X in some sense follows CPU 1's store to X and that CPU 2's store
to Y in some sense preceded CPU 3's load from Y. The question is then
"Can CPU 3's load from X return 0?"
Suppose that CPU 2's load from X returns 1, which it then stores to Y,
and CPU 3's load from Y returns 1. This indicates that CPU 1's store
to X precedes CPU 2's load from X and that CPU 2's store to Y precedes
CPU 3's load from Y. In addition, the memory barriers guarantee that
CPU 2 executes its load before its store, and CPU 3 loads from Y before
it loads from X. The question is then "Can CPU 3's load from X return 0?"
Because CPU 3's load from X in some sense came after CPU 2's load, it
Because CPU 3's load from X in some sense comes after CPU 2's load, it
is natural to expect that CPU 3's load from X must therefore return 1.
This expectation is an example of multicopy atomicity: if a load executing
on CPU A follows a load from the same variable executing on CPU B, then
an understandable but incorrect expectation is that CPU A's load must
either return the same value that CPU B's load did, or must return some
later value.
This expectation follows from multicopy atomicity: if a load executing
on CPU B follows a load from the same variable executing on CPU A (and
CPU A did not originally store the value which it read), then on
multicopy-atomic systems, CPU B's load must return either the same value
that CPU A's load did or some later value. However, the Linux kernel
does not require systems to be multicopy atomic.
In the Linux kernel, the above use of a general memory barrier compensates
for any lack of multicopy atomicity. Therefore, in the above example,
if CPU 2's load from X returns 1 and its load from Y returns 0, and CPU 3's
load from Y returns 1, then CPU 3's load from X must also return 1.
The use of a general memory barrier in the example above compensates
for any lack of multicopy atomicity. In the example, if CPU 2's load
from X returns 1 and CPU 3's load from Y returns 1, then CPU 3's load
from X must indeed also return 1.
However, dependencies, read barriers, and write barriers are not always
able to compensate for non-multicopy atomicity. For example, suppose
@ -1396,11 +1398,11 @@ this example, it is perfectly legal for CPU 2's load from X to return 1,
CPU 3's load from Y to return 1, and its load from X to return 0.
The key point is that although CPU 2's data dependency orders its load
and store, it does not guarantee to order CPU 1's store. Therefore,
if this example runs on a non-multicopy-atomic system where CPUs 1 and 2
share a store buffer or a level of cache, CPU 2 might have early access
to CPU 1's writes. A general barrier is therefore required to ensure
that all CPUs agree on the combined order of CPU 1's and CPU 2's accesses.
and store, it does not guarantee to order CPU 1's store. Thus, if this
example runs on a non-multicopy-atomic system where CPUs 1 and 2 share a
store buffer or a level of cache, CPU 2 might have early access to CPU 1's
writes. General barriers are therefore required to ensure that all CPUs
agree on the combined order of multiple accesses.
General barriers can compensate not only for non-multicopy atomicity,
but can also generate additional ordering that can ensure that -all-