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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2011-02-11 00:54:50 (GMT) |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2011-03-04 16:05:49 (GMT) |
commit | 241e6663b5151733294d1a230a3fd8a4d32e187f (patch) | |
tree | 088c44c088d13df65851a33f02f533ee3207335a | |
parent | e611eecd6f9f91d74beda8cfbb3b5e02abdeb5a1 (diff) | |
download | linux-241e6663b5151733294d1a230a3fd8a4d32e187f.tar.xz |
smp: Document transitivity for memory barriers.
Transitivity is guaranteed only for full memory barriers (smp_mb()).
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
-rw-r--r-- | Documentation/memory-barriers.txt | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 631ad2f..f0d3a80 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -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 ======================== |