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LITMUS TESTS
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============
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CoRR+poonceonce+Once.litmus
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Test of read-read coherence, that is, whether or not two
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successive reads from the same variable are ordered.
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CoRW+poonceonce+Once.litmus
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Test of read-write coherence, that is, whether or not a read
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from a given variable followed by a write to that same variable
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are ordered.
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CoWR+poonceonce+Once.litmus
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Test of write-read coherence, that is, whether or not a write
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to a given variable followed by a read from that same variable
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are ordered.
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CoWW+poonceonce.litmus
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Test of write-write coherence, that is, whether or not two
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successive writes to the same variable are ordered.
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IRIW+fencembonceonces+OnceOnce.litmus
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Test of independent reads from independent writes with smp_mb()
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between each pairs of reads. In other words, is smp_mb()
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sufficient to cause two different reading processes to agree on
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the order of a pair of writes, where each write is to a different
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variable by a different process? This litmus test is forbidden
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by LKMM's propagation rule.
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IRIW+poonceonces+OnceOnce.litmus
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Test of independent reads from independent writes with nothing
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between each pairs of reads. In other words, is anything at all
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needed to cause two different reading processes to agree on the
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order of a pair of writes, where each write is to a different
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variable by a different process?
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ISA2+pooncelock+pooncelock+pombonce.litmus
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Tests whether the ordering provided by a lock-protected S
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litmus test is visible to an external process whose accesses are
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separated by smp_mb(). This addition of an external process to
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S is otherwise known as ISA2.
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ISA2+poonceonces.litmus
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As below, but with store-release replaced with WRITE_ONCE()
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and load-acquire replaced with READ_ONCE().
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ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus
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Can a release-acquire chain order a prior store against
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a later load?
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LB+fencembonceonce+ctrlonceonce.litmus
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Does a control dependency and an smp_mb() suffice for the
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load-buffering litmus test, where each process reads from one
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of two variables then writes to the other?
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LB+poacquireonce+pooncerelease.litmus
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Does a release-acquire pair suffice for the load-buffering
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litmus test, where each process reads from one of two variables then
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writes to the other?
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LB+poonceonces.litmus
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As above, but with store-release replaced with WRITE_ONCE()
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and load-acquire replaced with READ_ONCE().
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MP+onceassign+derefonce.litmus
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As below, but with rcu_assign_pointer() and an rcu_dereference().
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MP+polockmbonce+poacquiresilsil.litmus
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Protect the access with a lock and an smp_mb__after_spinlock()
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in one process, and use an acquire load followed by a pair of
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spin_is_locked() calls in the other process.
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MP+polockonce+poacquiresilsil.litmus
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Protect the access with a lock in one process, and use an
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acquire load followed by a pair of spin_is_locked() calls
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in the other process.
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MP+polocks.litmus
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As below, but with the second access of the writer process
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and the first access of reader process protected by a lock.
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MP+poonceonces.litmus
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As below, but without the smp_rmb() and smp_wmb().
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MP+pooncerelease+poacquireonce.litmus
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As below, but with a release-acquire chain.
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MP+porevlocks.litmus
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As below, but with the first access of the writer process
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and the second access of reader process protected by a lock.
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MP+fencewmbonceonce+fencermbonceonce.litmus
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Does a smp_wmb() (between the stores) and an smp_rmb() (between
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the loads) suffice for the message-passing litmus test, where one
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process writes data and then a flag, and the other process reads
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the flag and then the data. (This is similar to the ISA2 tests,
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but with two processes instead of three.)
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R+fencembonceonces.litmus
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This is the fully ordered (via smp_mb()) version of one of
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the classic counterintuitive litmus tests that illustrates the
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effects of store propagation delays.
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R+poonceonces.litmus
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As above, but without the smp_mb() invocations.
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SB+fencembonceonces.litmus
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This is the fully ordered (again, via smp_mb() version of store
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buffering, which forms the core of Dekker's mutual-exclusion
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algorithm.
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SB+poonceonces.litmus
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As above, but without the smp_mb() invocations.
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SB+rfionceonce-poonceonces.litmus
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This litmus test demonstrates that LKMM is not fully multicopy
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atomic. (Neither is it other multicopy atomic.) This litmus test
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also demonstrates the "locations" debugging aid, which designates
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additional registers and locations to be printed out in the dump
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of final states in the herd7 output. Without the "locations"
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statement, only those registers and locations mentioned in the
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"exists" clause will be printed.
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S+poonceonces.litmus
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As below, but without the smp_wmb() and acquire load.
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S+fencewmbonceonce+poacquireonce.litmus
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Can a smp_wmb(), instead of a release, and an acquire order
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a prior store against a subsequent store?
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WRC+poonceonces+Once.litmus
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WRC+pooncerelease+fencermbonceonce+Once.litmus
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These two are members of an extension of the MP litmus-test
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class in which the first write is moved to a separate process.
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The second is forbidden because smp_store_release() is
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A-cumulative in LKMM.
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Z6.0+pooncelock+pooncelock+pombonce.litmus
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Is the ordering provided by a spin_unlock() and a subsequent
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spin_lock() sufficient to make ordering apparent to accesses
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by a process not holding the lock?
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Z6.0+pooncelock+poonceLock+pombonce.litmus
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As above, but with smp_mb__after_spinlock() immediately
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following the spin_lock().
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Z6.0+pooncerelease+poacquirerelease+fencembonceonce.litmus
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Is the ordering provided by a release-acquire chain sufficient
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to make ordering apparent to accesses by a process that does
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not participate in that release-acquire chain?
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A great many more litmus tests are available here:
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https://github.com/paulmckrcu/litmus
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==================
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LITMUS TEST NAMING
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==================
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Litmus tests are usually named based on their contents, which means that
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looking at the name tells you what the litmus test does. The naming
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scheme covers litmus tests having a single cycle that passes through
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each process exactly once, so litmus tests not fitting this description
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are named on an ad-hoc basis.
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The structure of a litmus-test name is the litmus-test class, a plus
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sign ("+"), and one string for each process, separated by plus signs.
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The end of the name is ".litmus".
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The litmus-test classes may be found in the infamous test6.pdf:
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https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf
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Each class defines the pattern of accesses and of the variables accessed.
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For example, if the one process writes to a pair of variables, and
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the other process reads from these same variables, the corresponding
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litmus-test class is "MP" (message passing), which may be found on the
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left-hand end of the second row of tests on page one of test6.pdf.
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The strings used to identify the actions carried out by each process are
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complex due to a desire to have short(er) names. Thus, there is a tool to
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generate these strings from a given litmus test's actions. For example,
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consider the processes from SB+rfionceonce-poonceonces.litmus:
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P0(int *x, int *y)
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{
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int r1;
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int r2;
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WRITE_ONCE(*x, 1);
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r1 = READ_ONCE(*x);
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r2 = READ_ONCE(*y);
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}
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P1(int *x, int *y)
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{
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int r3;
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int r4;
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WRITE_ONCE(*y, 1);
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r3 = READ_ONCE(*y);
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r4 = READ_ONCE(*x);
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}
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The next step is to construct a space-separated list of descriptors,
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interleaving descriptions of the relation between a pair of consecutive
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accesses with descriptions of the second access in the pair.
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P0()'s WRITE_ONCE() is read by its first READ_ONCE(), which is a
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reads-from link (rf) and internal to the P0() process. This is
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"rfi", which is an abbreviation for "reads-from internal". Because
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some of the tools string these abbreviations together with space
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characters separating processes, the first character is capitalized,
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resulting in "Rfi".
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P0()'s second access is a READ_ONCE(), as opposed to (for example)
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smp_load_acquire(), so next is "Once". Thus far, we have "Rfi Once".
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P0()'s third access is also a READ_ONCE(), but to y rather than x.
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This is related to P0()'s second access by program order ("po"),
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to a different variable ("d"), and both accesses are reads ("RR").
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The resulting descriptor is "PodRR". Because P0()'s third access is
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READ_ONCE(), we add another "Once" descriptor.
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A from-read ("fre") relation links P0()'s third to P1()'s first
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access, and the resulting descriptor is "Fre". P1()'s first access is
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WRITE_ONCE(), which as before gives the descriptor "Once". The string
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thus far is thus "Rfi Once PodRR Once Fre Once".
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The remainder of P1() is similar to P0(), which means we add
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"Rfi Once PodRR Once". Another fre links P1()'s last access to
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P0()'s first access, which is WRITE_ONCE(), so we add "Fre Once".
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The full string is thus:
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Rfi Once PodRR Once Fre Once Rfi Once PodRR Once Fre Once
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This string can be given to the "norm7" and "classify7" tools to
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produce the name:
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$ norm7 -bell linux-kernel.bell \
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Rfi Once PodRR Once Fre Once Rfi Once PodRR Once Fre Once | \
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sed -e 's/:.*//g'
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SB+rfionceonce-poonceonces
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Adding the ".litmus" suffix: SB+rfionceonce-poonceonces.litmus
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The descriptors that describe connections between consecutive accesses
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within the cycle through a given litmus test can be provided by the herd7
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tool (Rfi, Po, Fre, and so on) or by the linux-kernel.bell file (Once,
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Release, Acquire, and so on).
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To see the full list of descriptors, execute the following command:
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$ diyone7 -bell linux-kernel.bell -show edges
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