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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 10:01:02AM -0800, Paul E. McKenney wrote:
> 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.
>

Looks good to me, thanks!

Regards,
Boqun

> 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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