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smp: Document transitivity for memory barriers.

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Transitivity is guaranteed only for full memory barriers (smp_mb()).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Paul E. McKenney 2011-02-10 16:54:50 -08:00
parent e611eecd6f
commit 241e6663b5
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Documentation/memory-barriers.txt
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@ -21,6 +21,7 @@ Contents:
- SMP barrier pairing.
- Examples of memory barrier sequences.
- Read memory barriers vs load speculation.
- Transitivity
(*) Explicit kernel barriers.
@ -959,6 +960,63 @@ the speculation will be cancelled and the value reloaded:
retrieved : : +-------+
TRANSITIVITY
------------
Transitivity is a deeply intuitive notion about ordering that is not
always provided by real computer systems. The following example
demonstrates transitivity (also called "cumulativity"):
CPU 1 CPU 2 CPU 3
======================= ======================= =======================
{ X = 0, Y = 0 }
STORE X=1 LOAD X STORE Y=1
<general barrier> <general barrier>
LOAD Y LOAD X
Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
This indicates that CPU 2's load from X in some sense follows CPU 1's
store to X and that CPU 2's load from Y in some sense preceded CPU 3's
store to Y. The question is then "Can CPU 3's load from X return 0?"
Because CPU 2's load from X in some sense came after CPU 1's store, it
is natural to expect that CPU 3's load from X must therefore return 1.
This expectation is an example of transitivity: if a load executing on
CPU A follows a load from the same variable executing on CPU B, then
CPU A's load must either return the same value that CPU B's load did,
or must return some later value.
In the Linux kernel, use of general memory barriers guarantees
transitivity. Therefore, in the above example, if CPU 2's load from X
returns 1 and its load from Y returns 0, then CPU 3's load from X must
also return 1.
However, transitivity is -not- guaranteed for read or write barriers.
For example, suppose that CPU 2's general barrier in the above example
is changed to a read barrier as shown below:
CPU 1 CPU 2 CPU 3
======================= ======================= =======================
{ X = 0, Y = 0 }
STORE X=1 LOAD X STORE Y=1
<read barrier> <general barrier>
LOAD Y LOAD X
This substitution destroys transitivity: in this example, it is perfectly
legal for CPU 2's load from X to return 1, its load from Y to return 0,
and CPU 3's load from X to return 0.
The key point is that although CPU 2's read barrier orders its pair
of loads, it does not guarantee to order CPU 1's store. Therefore, if
this example runs on a 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 CPU 1's and CPU 2's accesses.
To reiterate, if your code requires transitivity, use general barriers
throughout.
========================
EXPLICIT KERNEL BARRIERS
========================