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This document contains brief definitions of LKMM-related terms. Like most
glossaries, it is not intended to be read front to back (except perhaps
as a way of confirming a diagnosis of OCD), but rather to be searched
for specific terms.
Address Dependency: When the address of a later memory access is computed
based on the value returned by an earlier load, an "address
dependency" extends from that load extending to the later access.
Address dependencies are quite common in RCU read-side critical
sections:
1 rcu_read_lock();
2 p = rcu_dereference(gp);
3 do_something(p->a);
4 rcu_read_unlock();
In this case, because the address of "p->a" on line 3 is computed
from the value returned by the rcu_dereference() on line 2, the
address dependency extends from that rcu_dereference() to that
"p->a". In rare cases, optimizing compilers can destroy address
dependencies. Please see Documentation/RCU/rcu_dereference.rst
for more information.
See also "Control Dependency" and "Data Dependency".
Acquire: With respect to a lock, acquiring that lock, for example,
using spin_lock(). With respect to a non-lock shared variable,
a special operation that includes a load and which orders that
load before later memory references running on that same CPU.
An example special acquire operation is smp_load_acquire(),
but atomic_read_acquire() and atomic_xchg_acquire() also include
acquire loads.
When an acquire load returns the value stored by a release store
to that same variable, (in other words, the acquire load "reads
from" the release store), then all operations preceding that
store "happen before" any operations following that load acquire.
See also "Happens-Before", "Reads-From", "Relaxed", and "Release".
Coherence (co): When one CPU's store to a given variable overwrites
either the value from another CPU's store or some later value,
there is said to be a coherence link from the second CPU to
the first.
It is also possible to have a coherence link within a CPU, which
is a "coherence internal" (coi) link. The term "coherence
external" (coe) link is used when it is necessary to exclude
the coi case.
See also "From-reads" and "Reads-from".
Control Dependency: When a later store's execution depends on a test
of a value computed from a value returned by an earlier load,
a "control dependency" extends from that load to that store.
For example:
1 if (READ_ONCE(x))
2 WRITE_ONCE(y, 1);
Here, the control dependency extends from the READ_ONCE() on
line 1 to the WRITE_ONCE() on line 2. Control dependencies are
fragile, and can be easily destroyed by optimizing compilers.
Please see control-dependencies.txt for more information.
See also "Address Dependency" and "Data Dependency".
Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the
name suggests. And in a great many cases, a pair of CPUs is all
that is required. In other cases, the notion of pairing must be
extended to additional CPUs, and the result is called a "cycle".
In a cycle, each CPU's ordering interacts with that of the next:
CPU 0 CPU 1 CPU 2
WRITE_ONCE(x, 1); WRITE_ONCE(y, 1); WRITE_ONCE(z, 1);
smp_mb(); smp_mb(); smp_mb();
r0 = READ_ONCE(y); r1 = READ_ONCE(z); r2 = READ_ONCE(x);
CPU 0's smp_mb() interacts with that of CPU 1, which interacts
with that of CPU 2, which in turn interacts with that of CPU 0
to complete the cycle. Because of the smp_mb() calls between
each pair of memory accesses, the outcome where r0, r1, and r2
are all equal to zero is forbidden by LKMM.
See also "Pairing".
Data Dependency: When the data written by a later store is computed based
on the value returned by an earlier load, a "data dependency"
extends from that load to that later store. For example:
1 r1 = READ_ONCE(x);
2 WRITE_ONCE(y, r1 + 1);
In this case, the data dependency extends from the READ_ONCE()
on line 1 to the WRITE_ONCE() on line 2. Data dependencies are
fragile and can be easily destroyed by optimizing compilers.
Because optimizing compilers put a great deal of effort into
working out what values integer variables might have, this is
especially true in cases where the dependency is carried through
an integer.
See also "Address Dependency" and "Control Dependency".
From-Reads (fr): When one CPU's store to a given variable happened
too late to affect the value returned by another CPU's
load from that same variable, there is said to be a from-reads
link from the load to the store.
It is also possible to have a from-reads link within a CPU, which
is a "from-reads internal" (fri) link. The term "from-reads
external" (fre) link is used when it is necessary to exclude
the fri case.
See also "Coherence" and "Reads-from".
Fully Ordered: An operation such as smp_mb() that orders all of
its CPU's prior accesses with all of that CPU's subsequent
accesses, or a marked access such as atomic_add_return()
that orders all of its CPU's prior accesses, itself, and
all of its CPU's subsequent accesses.
Happens-Before (hb): A relation between two accesses in which LKMM
guarantees the first access precedes the second. For more
detail, please see the "THE HAPPENS-BEFORE RELATION: hb"
section of explanation.txt.
Marked Access: An access to a variable that uses an special function or
macro such as "r1 = READ_ONCE(x)" or "smp_store_release(&a, 1)".
See also "Unmarked Access".
Pairing: "Memory-barrier pairing" reflects the fact that synchronizing
data between two CPUs requires that both CPUs their accesses.
Memory barriers thus tend to come in pairs, one executed by
one of the CPUs and the other by the other CPU. Of course,
pairing also occurs with other types of operations, so that a
smp_store_release() pairs with an smp_load_acquire() that reads
the value stored.
See also "Cycle".
Reads-From (rf): When one CPU's load returns the value stored by some other
CPU, there is said to be a reads-from link from the second
CPU's store to the first CPU's load. Reads-from links have the
nice property that time must advance from the store to the load,
which means that algorithms using reads-from links can use lighter
weight ordering and synchronization compared to algorithms using
coherence and from-reads links.
It is also possible to have a reads-from link within a CPU, which
is a "reads-from internal" (rfi) link. The term "reads-from
external" (rfe) link is used when it is necessary to exclude
the rfi case.
See also Coherence" and "From-reads".
Relaxed: A marked access that does not imply ordering, for example, a
READ_ONCE(), WRITE_ONCE(), a non-value-returning read-modify-write
operation, or a value-returning read-modify-write operation whose
name ends in "_relaxed".
See also "Acquire" and "Release".
Release: With respect to a lock, releasing that lock, for example,
using spin_unlock(). With respect to a non-lock shared variable,
a special operation that includes a store and which orders that
store after earlier memory references that ran on that same CPU.
An example special release store is smp_store_release(), but
atomic_set_release() and atomic_cmpxchg_release() also include
release stores.
See also "Acquire" and "Relaxed".
Unmarked Access: An access to a variable that uses normal C-language
syntax, for example, "a = b[2]";
See also "Marked Access".
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