On Fri, Nov 06, 2020 at 09:47:22AM +0800, Boqun Feng wrote:
> On Thu, Nov 05, 2020 at 02:00:14PM -0800, [email protected] wrote:
> > From: "Paul E. McKenney" <[email protected]>
> >
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > ---
> > 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 <[email protected]>
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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
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)".
