locking will be seen as ordered by CPUs not holding that lock.
Consider this example:
- /* See Z6.0+pooncelock+pooncelock+pombonce.litmus. */
+ /* See Z6.0+pooncerelease+poacquirerelease+fencembonceonce.litmus. */
void CPU0(void)
{
spin_lock(&mylock);
smp_wmb() and smp_rmb() APIs are still heavily used, so it is important
to understand their use cases. The general approach is shown below:
- /* See MP+wmbonceonce+rmbonceonce.litmus. */
+ /* See MP+fencewmbonceonce+fencermbonceonce.litmus. */
void CPU0(void)
{
WRITE_ONCE(x, 1);
One way of avoiding the counter-intuitive outcome is through the use of a
control dependency paired with a full memory barrier:
- /* See LB+ctrlonceonce+mbonceonce.litmus. */
+ /* See LB+fencembonceonce+ctrlonceonce.litmus. */
void CPU0(void)
{
r0 = READ_ONCE(x);
while another CPU stores to the second variable and then loads from the
first. Preserving order requires nothing less than full barriers:
- /* See SB+mbonceonces.litmus. */
+ /* See SB+fencembonceonces.litmus. */
void CPU0(void)
{
WRITE_ONCE(x, 1);
The memory model is used, in conjunction with "herd7", to exhaustively
explore the state space of small litmus tests.
-For example, to run SB+mbonceonces.litmus against the memory model:
+For example, to run SB+fencembonceonces.litmus against the memory model:
- $ herd7 -conf linux-kernel.cfg litmus-tests/SB+mbonceonces.litmus
+ $ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
Here is the corresponding output:
- Test SB+mbonceonces Allowed
+ Test SB+fencembonceonces Allowed
States 3
0:r0=0; 1:r0=1;
0:r0=1; 1:r0=0;
Witnesses
Positive: 0 Negative: 3
Condition exists (0:r0=0 /\ 1:r0=0)
- Observation SB+mbonceonces Never 0 3
- Time SB+mbonceonces 0.01
+ Observation SB+fencembonceonces Never 0 3
+ Time SB+fencembonceonces 0.01
Hash=d66d99523e2cac6b06e66f4c995ebb48
The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
The "klitmus7" tool converts a litmus test into a Linux kernel module,
which may then be loaded and run.
-For example, to run SB+mbonceonces.litmus against hardware:
+For example, to run SB+fencembonceonces.litmus against hardware:
$ mkdir mymodules
- $ klitmus7 -o mymodules litmus-tests/SB+mbonceonces.litmus
+ $ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus
$ cd mymodules ; make
$ sudo sh run.sh
The corresponding output includes:
- Test SB+mbonceonces Allowed
+ Test SB+fencembonceonces Allowed
Histogram (3 states)
644580 :>0:r0=1; 1:r0=0;
644328 :>0:r0=0; 1:r0=1;
Positive: 0, Negative: 2000000
Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
Hash=d66d99523e2cac6b06e66f4c995ebb48
- Observation SB+mbonceonces Never 0 2000000
- Time SB+mbonceonces 0.16
+ Observation SB+fencembonceonces Never 0 2000000
+ Time SB+fencembonceonces 0.16
The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
that during two million trials, the state specified in this litmus
Test of write-write coherence, that is, whether or not two
successive writes to the same variable are ordered.
-IRIW+mbonceonces+OnceOnce.litmus
+IRIW+fencembonceonces+OnceOnce.litmus
Test of independent reads from independent writes with smp_mb()
between each pairs of reads. In other words, is smp_mb()
sufficient to cause two different reading processes to agree on
Can a release-acquire chain order a prior store against
a later load?
-LB+ctrlonceonce+mbonceonce.litmus
+LB+fencembonceonce+ctrlonceonce.litmus
Does a control dependency and an smp_mb() suffice for the
load-buffering litmus test, where each process reads from one
of two variables then writes to the other?
As below, but with the first access of the writer process
and the second access of reader process protected by a lock.
-MP+wmbonceonce+rmbonceonce.litmus
+MP+fencewmbonceonce+fencermbonceonce.litmus
Does a smp_wmb() (between the stores) and an smp_rmb() (between
the loads) suffice for the message-passing litmus test, where one
process writes data and then a flag, and the other process reads
the flag and then the data. (This is similar to the ISA2 tests,
but with two processes instead of three.)
-R+mbonceonces.litmus
+R+fencembonceonces.litmus
This is the fully ordered (via smp_mb()) version of one of
the classic counterintuitive litmus tests that illustrates the
effects of store propagation delays.
R+poonceonces.litmus
As above, but without the smp_mb() invocations.
-SB+mbonceonces.litmus
+SB+fencembonceonces.litmus
This is the fully ordered (again, via smp_mb() version of store
buffering, which forms the core of Dekker's mutual-exclusion
algorithm.
S+poonceonces.litmus
As below, but without the smp_wmb() and acquire load.
-S+wmbonceonce+poacquireonce.litmus
+S+fencewmbonceonce+poacquireonce.litmus
Can a smp_wmb(), instead of a release, and an acquire order
a prior store against a subsequent store?
WRC+poonceonces+Once.litmus
-WRC+pooncerelease+rmbonceonce+Once.litmus
+WRC+pooncerelease+fencermbonceonce+Once.litmus
These two are members of an extension of the MP litmus-test
class in which the first write is moved to a separate process.
The second is forbidden because smp_store_release() is
As above, but with smp_mb__after_spinlock() immediately
following the spin_lock().
-Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus
+Z6.0+pooncerelease+poacquirerelease+fencembonceonce.litmus
Is the ordering provided by a release-acquire chain sufficient
to make ordering apparent to accesses by a process that does
not participate in that release-acquire chain?
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