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    SubjectRe: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms
    On Fri, Nov 06, 2020 at 09:47:22AM +0800, Boqun Feng wrote:
    > On Thu, Nov 05, 2020 at 02:00:14PM -0800, paulmck@kernel.org wrote:
    > > From: "Paul E. McKenney" <paulmck@kernel.org>
    > >
    > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    > > ---
    > > tools/memory-model/Documentation/glossary.txt | 155 ++++++++++++++++++++++++++
    > > 1 file changed, 155 insertions(+)
    > > create mode 100644 tools/memory-model/Documentation/glossary.txt
    > >
    > > diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt
    > > new file mode 100644
    > > index 0000000..036fa28
    > > --- /dev/null
    > > +++ b/tools/memory-model/Documentation/glossary.txt
    > > @@ -0,0 +1,155 @@
    > > +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.txt
    > > + for more information.
    > > +
    > > + See also "Control 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, then all operations preceding that store
    >
    > Change this to:
    >
    > When an acquire load reads-from a release store
    >
    > , and put a reference to "Reads-from"? I think this makes the document
    > more consistent in that it makes clear "an acquire load returns the
    > value stored by a release store to the same variable" is not a special
    > case, it's simple a "Reads-from".
    >
    > > + happen before any operations following that load acquire.
    >
    > Add a reference to the definition of "happen before" in explanation.txt?

    How about as shown below? I currently am carrying this as a separate
    commit, but I might merge it into this one later on.

    Thanx, Paul

    ------------------------------------------------------------------------

    commit 774a52cd3d80d6b657ae6c14c10bd9fc437068f3
    Author: Paul E. McKenney <paulmck@kernel.org>
    Date: Fri Nov 6 09:58:01 2020 -0800

    tools/memory-model: Tie acquire loads to reads-from

    This commit explicitly makes the connection between acquire loads and
    the reads-from relation. It also adds an entry for happens-before,
    and refers to the corresponding section of explanation.txt.

    Reported-by: Boqun Feng <boqun.feng@gmail.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

    diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt
    index 3924aca..383151b 100644
    --- a/tools/memory-model/Documentation/glossary.txt
    +++ b/tools/memory-model/Documentation/glossary.txt
    @@ -33,10 +33,11 @@ Acquire: With respect to a lock, acquiring that lock, for example,
    acquire loads.

    When an acquire load returns the value stored by a release store
    - to that same variable, then all operations preceding that store
    - happen before any operations following that load acquire.
    + 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 "Relaxed" and "Release".
    + 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,
    @@ -102,6 +103,11 @@ Fully Ordered: An operation such as smp_mb() that orders all of
    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()" or "smp_store_release(&a, 1)".

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