xref: /linux/tools/memory-model/litmus-tests/README (revision c4f790f244070dbab486805276ba4d1f87a057af)

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LITMUS TESTS
============

CoRR+poonceonce+Once.litmus
	Test of read-read coherence, that is, whether or not two
	successive reads from the same variable are ordered.

CoRW+poonceonce+Once.litmus
	Test of read-write coherence, that is, whether or not a read
	from a given variable followed by a write to that same variable
	are ordered.

CoWR+poonceonce+Once.litmus
	Test of write-read coherence, that is, whether or not a write
	to a given variable followed by a read from that same variable
	are ordered.

CoWW+poonceonce.litmus
	Test of write-write coherence, that is, whether or not two
	successive writes to the same variable are ordered.

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
	the order of a pair of writes, where each write is to a different
	variable by a different process?  This litmus test is forbidden
	by LKMM's propagation rule.

IRIW+poonceonces+OnceOnce.litmus
	Test of independent reads from independent writes with nothing
	between each pairs of reads.  In other words, is anything at all
	needed to cause two different reading processes to agree on the
	order of a pair of writes, where each write is to a different
	variable by a different process?

ISA2+pooncelock+pooncelock+pombonce.litmus
	Tests whether the ordering provided by a lock-protected S
	litmus test is visible to an external process whose accesses are
	separated by smp_mb().  This addition of an external process to
	S is otherwise known as ISA2.

ISA2+poonceonces.litmus
	As below, but with store-release replaced with WRITE_ONCE()
	and load-acquire replaced with READ_ONCE().

ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus
	Can a release-acquire chain order a prior store against
	a later load?

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?

LB+poacquireonce+pooncerelease.litmus
	Does a release-acquire pair suffice for the load-buffering
	litmus test, where each process reads from one of two variables then
	writes to the other?

LB+poonceonces.litmus
	As above, but with store-release replaced with WRITE_ONCE()
	and load-acquire replaced with READ_ONCE().

MP+onceassign+derefonce.litmus
	As below, but with rcu_assign_pointer() and an rcu_dereference().

MP+polockmbonce+poacquiresilsil.litmus
	Protect the access with a lock and an smp_mb__after_spinlock()
	in one process, and use an acquire load followed by a pair of
	spin_is_locked() calls in the other process.

MP+polockonce+poacquiresilsil.litmus
	Protect the access with a lock in one process, and use an
	acquire load followed by a pair of spin_is_locked() calls
	in the other process.

MP+polocks.litmus
	As below, but with the second access of the writer process
	and the first access of reader process protected by a lock.

MP+poonceonces.litmus
	As below, but without the smp_rmb() and smp_wmb().

MP+pooncerelease+poacquireonce.litmus
	As below, but with a release-acquire chain.

MP+porevlocks.litmus
	As below, but with the first access of the writer process
	and the second access of reader process protected by a lock.

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+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+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.

SB+poonceonces.litmus
	As above, but without the smp_mb() invocations.

SB+rfionceonce-poonceonces.litmus
	This litmus test demonstrates that LKMM is not fully multicopy
	atomic.  (Neither is it other multicopy atomic.)  This litmus test
	also demonstrates the "locations" debugging aid, which designates
	additional registers and locations to be printed out in the dump
	of final states in the herd7 output.  Without the "locations"
	statement, only those registers and locations mentioned in the
	"exists" clause will be printed.

S+poonceonces.litmus
	As below, but without the smp_wmb() and acquire load.

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+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
	A-cumulative in LKMM.

Z6.0+pooncelock+pooncelock+pombonce.litmus
	Is the ordering provided by a spin_unlock() and a subsequent
	spin_lock() sufficient to make ordering apparent to accesses
	by a process not holding the lock?

Z6.0+pooncelock+poonceLock+pombonce.litmus
	As above, but with smp_mb__after_spinlock() immediately
	following the spin_lock().

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?

A great many more litmus tests are available here:

	https://github.com/paulmckrcu/litmus

==================
LITMUS TEST NAMING
==================

Litmus tests are usually named based on their contents, which means that
looking at the name tells you what the litmus test does.  The naming
scheme covers litmus tests having a single cycle that passes through
each process exactly once, so litmus tests not fitting this description
are named on an ad-hoc basis.

The structure of a litmus-test name is the litmus-test class, a plus
sign ("+"), and one string for each process, separated by plus signs.
The end of the name is ".litmus".

The litmus-test classes may be found in the infamous test6.pdf:
https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf
Each class defines the pattern of accesses and of the variables accessed.
For example, if the one process writes to a pair of variables, and
the other process reads from these same variables, the corresponding
litmus-test class is "MP" (message passing), which may be found on the
left-hand end of the second row of tests on page one of test6.pdf.

The strings used to identify the actions carried out by each process are
complex due to a desire to have short(er) names.  Thus, there is a tool to
generate these strings from a given litmus test's actions.  For example,
consider the processes from SB+rfionceonce-poonceonces.litmus:

	P0(int *x, int *y)
	{
		int r1;
		int r2;

		WRITE_ONCE(*x, 1);
		r1 = READ_ONCE(*x);
		r2 = READ_ONCE(*y);
	}

	P1(int *x, int *y)
	{
		int r3;
		int r4;

		WRITE_ONCE(*y, 1);
		r3 = READ_ONCE(*y);
		r4 = READ_ONCE(*x);
	}

The next step is to construct a space-separated list of descriptors,
interleaving descriptions of the relation between a pair of consecutive
accesses with descriptions of the second access in the pair.

P0()'s WRITE_ONCE() is read by its first READ_ONCE(), which is a
reads-from link (rf) and internal to the P0() process.  This is
"rfi", which is an abbreviation for "reads-from internal".  Because
some of the tools string these abbreviations together with space
characters separating processes, the first character is capitalized,
resulting in "Rfi".

P0()'s second access is a READ_ONCE(), as opposed to (for example)
smp_load_acquire(), so next is "Once".  Thus far, we have "Rfi Once".

P0()'s third access is also a READ_ONCE(), but to y rather than x.
This is related to P0()'s second access by program order ("po"),
to a different variable ("d"), and both accesses are reads ("RR").
The resulting descriptor is "PodRR".  Because P0()'s third access is
READ_ONCE(), we add another "Once" descriptor.

A from-read ("fre") relation links P0()'s third to P1()'s first
access, and the resulting descriptor is "Fre".  P1()'s first access is
WRITE_ONCE(), which as before gives the descriptor "Once".  The string
thus far is thus "Rfi Once PodRR Once Fre Once".

The remainder of P1() is similar to P0(), which means we add
"Rfi Once PodRR Once".  Another fre links P1()'s last access to
P0()'s first access, which is WRITE_ONCE(), so we add "Fre Once".
The full string is thus:

	Rfi Once PodRR Once Fre Once Rfi Once PodRR Once Fre Once

This string can be given to the "norm7" and "classify7" tools to
produce the name:

	$ norm7 -bell linux-kernel.bell \
		Rfi Once PodRR Once Fre Once Rfi Once PodRR Once Fre Once | \
	  sed -e 's/:.*//g'
	SB+rfionceonce-poonceonces

Adding the ".litmus" suffix: SB+rfionceonce-poonceonces.litmus

The descriptors that describe connections between consecutive accesses
within the cycle through a given litmus test can be provided by the herd
tool (Rfi, Po, Fre, and so on) or by the linux-kernel.bell file (Once,
Release, Acquire, and so on).

To see the full list of descriptors, execute the following command:

	$ diyone7 -bell linux-kernel.bell -show edges