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Subject[PATCH v1 0/2] docs/memory-barriers.txt: Fix confusing name of 'data dependency barrier'
Hi all,

This is a revised patch set of RFC [1].

Discussion so far is about possible follow-up improvements,
so I hereby submit this set as a "v1".

Changes since RFC [1]:

- Rename title of Patch 1/2.
- Remove note on the rename of section "DATA DEPENDENCY BARRIER".
Rational in the changelog should suffice.
- Wordsmith by self review.
- Add Patch 2/2 (fixup of long lines).

[1]: https://lore.kernel.org/linux-doc/cc2c7885-ac75-24f3-e18a-e77f97c91b4c@gmail.com/ # RFC

For your convenience, diff of "v1 1/2" vs RFC is appended below.

Following is the explanation of background in RFC (with typo fixes):
-------------------------------------------------------------------
This is a response to Michael's report back in last November [2].

[2]: "data dependency naming inconsistency":
https://lore.kernel.org/r/20211011064233-mutt-send-email-mst@kernel.org/

In the thread, I suggested removing all the explanations of "data dependency
barriers", which Paul thought was reasonable.

However, such removal would require involved rewrites in the infamously
hard-to-grasp document, which is beyond my capability.
I have become more inclined to just substitute "data dependency barrier"
with "address-dependency barrier" considering the that READ_ONCE() still
has an implicit address-dependency barrier.

This patch set is the result of such an attempt.

Note: I made a mistake in the thread above. Kernel APIs for explicit data
dependency barriers were removed in v5.9.
I was confused the removal with the addition of the barrier to Alpha's
READ_ONCE() in v4.15.

diff of "v1 1/2" vs RFC
------------------------------------------------------------------
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 306afa1f9347..bdbea3cc66a3 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -391,8 +391,8 @@ Memory barriers come in four basic varieties:
memory system as time progresses. All stores _before_ a write barrier
will occur _before_ all the stores after the write barrier.

- [!] Note that write barriers should normally be paired with read- or address-
- dependency barriers; see the "SMP barrier pairing" subsection.
+ [!] Note that write barriers should normally be paired with read or
+ address-dependency barriers; see the "SMP barrier pairing" subsection.


(2) Address-dependency barriers (historical).
@@ -561,17 +561,14 @@ As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
DEC Alpha, which means that about the only people who need to pay attention
to this section are those working on DEC Alpha architecture-specific code
and those working on READ_ONCE() itself. For those who need it, and for
-those who are interested in the history, here is the story of address-
-dependency barriers.
+those who are interested in the history, here is the story of
+address-dependency barriers.

-[!] The title of this section was renamed from "DATA DEPENDENCY BARRIERS"
-to prevent developer confusion as "data dependencies" usually refers to
-load-to-store data dependencies.
-While address dependencies are observed in both load-to-load and load-to-
-store relations, address-dependency barriers concern only load-to-load
-situations.
+[!] While address dependencies are observed in both load-to-load and
+load-to-store relations, address-dependency barriers are not necessary
+for load-to-store situations.

-The usage requirements of address-dependency barriers are a little subtle, and
+The requirement of address-dependency barriers is a little subtle, and
it's not always obvious that they're needed. To illustrate, consider the
following sequence of events:

@@ -602,8 +599,8 @@ While this may seem like a failure of coherency or causality maintenance, it
isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
Alpha).

-To deal with this, an implicit address-dependency barrier of READ_ONCE()
-or better must be inserted between the address load and the data load:
+To deal with this, READ_ONCE() provides an implicit address-dependency
+barrier since kernel release v4.15:

CPU 1 CPU 2
=============== ===============
@@ -659,11 +656,9 @@ can be used to record rare error conditions and the like, and the CPUs'
naturally occurring ordering prevents such records from being lost.


-Note well that the ordering provided by an address or a data dependency is local to
+Note well that the ordering provided by an address dependency is local to
the CPU containing it. See the section on "Multicopy atomicity" for
more information.

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

Thanks, Akira
--
Akira Yokosawa (2):
docs/memory-barriers.txt: Fix confusing name of 'data dependency
barrier'
docs/memory-barriers.txt: Fixup long lines
Documentation/memory-barriers.txt | 177 ++++++++++++++++--------------
1 file changed, 95 insertions(+), 82 deletions(-)

base-commit: 21710a691d770f8b48e2de66426fb1c1c8416ee6
--
2.25.1

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