Re: comments style: Re: [RFC PATCH v4 1/9] printk-rb: add a new printk ringbuffer implementation

Sergey Senozhatsky
2019-08-23T05:54:53Z

On (08/21/19 07:46), John Ogness wrote:
[..]
> The labels are necessary for the technical documentation of the
> barriers. And, after spending much time in this, I find them very
> useful. But I agree that there needs to be a better way to assign label
> names.
[..]
> > Where dp stands for descriptor push. For dataring we can add a 'dr'
> > prefix, to avoid confusion with desc barriers, which have 'd' prefix.
> > And so on. Dunno.
>
> Yeah, I spent a lot of time going in circles on this one.
[..]
> I hope that we can agree that the labels are important. And that a
> formal documentation of the barriers is also important. Yes, they are a
> lot of work, but I find it makes it a lot easier to go back to the code
> after I've been away for a while. Even now, as I go through your
> feedback on code that I wrote over a month ago, I find the formal
> comments critical to quickly understand _exactly_ why the memory
> barriers exist.

Yeah. I like those tagsi/labels, and appreciate your efforts.

Speaking about it in general, not necessarily related to printk patch set.
With or without labels/tags we still have to grep. But grep-ing is much
easier when we have labels/tags. Otherwise it's sometimes hard to understand
what to grep for - _acquire, _relaxed, smp barrier, write_once, or
anything else.

> Perhaps we should choose labels that are more clear, like:
>
> dataring_push:A
> dataring_push:B
>
> Then we would see comments like:
>
> Memory barrier involvement:
>
> If _dataring_pop:B reads from dataring_datablock_setid:A, then
> _dataring_pop:C reads from dataring_push:G.
[..]
> RELEASE from dataring_push:E to dataring_datablock_setid:A
> matching
> ACQUIRE from _dataring_pop:B to _dataring_pop:E

I thought about it. That's very informative, albeit pretty hard to maintain.
The same applies to drA or prA and any other context dependent prefix.

> But then how should the labels in the code look? Just the letter looks
> simple in code, but cannot be grepped.

Yes, good point.

> dataring_push()
> {
> ...
> /* E */
> ...
> }

If only there was something as cool as grep-ing, but cooler. Something
that "just sucks less". Something that even folks like myself could use.

Bare with me.

Apologies. This email is rather long; but it's pretty easy to read.
Let's see if this can fly.

So what I did.

I changed several LMM tags/labels definitions, so they have common format:

LMM_TAG(name)

I don't insist on this particular naming scheme, it can be improved.

======================================================================
diff --git a/kernel/printk/dataring.c b/kernel/printk/dataring.c
index e48069dc27bc..54eb28d47d30 100644
--- a/kernel/printk/dataring.c
+++ b/kernel/printk/dataring.c
@@ -577,11 +577,11 @@ char *dataring_push(struct dataring *dr, unsigned long size,
to_db_size(&size);

do {
- /* fA: */
+ /* LMM_TAG(fA) */
ret = get_new_lpos(dr, size, &begin_lpos, &next_lpos);

/*
- * fB:
+ * LMM_TAG(fB)
*
* The data ringbuffer tail may have been pushed (by this or
* any other task). The updated @tail_lpos must be visible to
@@ -621,7 +621,7 @@ char *dataring_push(struct dataring *dr, unsigned long size,

if (!ret) {
/*
- * fC:
+ * LMM_TAG(fC)
*
* Force @desc permanently invalid to minimize risk
* of the descriptor later unexpectedly being
@@ -631,14 +631,14 @@ char *dataring_push(struct dataring *dr, unsigned long size,
dataring_desc_init(desc);
return NULL;
}
- /* fE: */
+ /* LMM_TAG(fE) */
} while (atomic_long_cmpxchg_relaxed(&dr->head_lpos, begin_lpos,
next_lpos) != begin_lpos);

db = to_datablock(dr, begin_lpos);

/*
- * fF:
+ * LMM_TAG(fF)
*
* @db->id is a garbage value and could possibly match the @id. This
* would be a problem because the data block would be considered
@@ -648,7 +648,7 @@ char *dataring_push(struct dataring *dr, unsigned long size,
WRITE_ONCE(db->id, id - 1);

/*
- * fG:
+ * LMM_TAG(fG)
*
* Ensure that @db->id is initialized to a wrong ID value before
* setting @begin_lpos so that there is no risk of accidentally
@@ -668,7 +668,7 @@ char *dataring_push(struct dataring *dr, unsigned long size,
*/
smp_store_release(&desc->begin_lpos, begin_lpos);

- /* fH: */
+ /* LMM_TAG(fH) */
WRITE_ONCE(desc->next_lpos, next_lpos);

/* If this data block wraps, use @data from the content data block. */
diff --git a/kernel/printk/numlist.c b/kernel/printk/numlist.c
index 16c6ffa74b01..285e0431dbf8 100644
--- a/kernel/printk/numlist.c
+++ b/kernel/printk/numlist.c
@@ -338,7 +338,7 @@ struct nl_node *numlist_pop(struct numlist *nl)
tail_id = atomic_long_read(&nl->tail_id);

for (;;) {
- /* cB */
+ /* LMM_TAG(cB) */
while (!numlist_read(nl, tail_id, NULL, &next_id)) {
/*
* @tail_id is invalid. Try again with an
@@ -357,6 +357,7 @@ struct nl_node *numlist_pop(struct numlist *nl)

/*
* cC:
+ * LMM_TAG(cD)
*
* Make sure the node is not busy.
*/
@@ -368,7 +369,7 @@ struct nl_node *numlist_pop(struct numlist *nl)
if (r == tail_id)
break;

- /* cA: #3 */
+ /* LMM_TAG(cA) #3 */
tail_id = r;
}

======================================================================

Okay.
Next, I added the following simple quick-n-dirty perl script:

======================================================================
Subject: [PATCH] add LMM_TAG parser

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
scripts/ctags-parse-lmm.pl | 45 ++++++++++++++++++++++++++++++++++++++
1 file changed, 45 insertions(+)
create mode 100755 scripts/ctags-parse-lmm.pl

diff --git a/scripts/ctags-parse-lmm.pl b/scripts/ctags-parse-lmm.pl
new file mode 100755
index 000000000000..785f6945c936
--- /dev/null
+++ b/scripts/ctags-parse-lmm.pl
@@ -0,0 +1,45 @@
+#!/usr/bin/perl
+#
+# SPDX-License-Identifier: GPL-2.0
+#
+# Parse Linux Memory Model tags and add corresponding entries to the ctags file
+#
+# LMM Über Alles!
+
+use strict;
+
+sub parse($$)
+{
+ my ($t, $f) = @_;
+ my $ctags;
+ my $file;
+
+ if (!open($ctags, '>>', $t)) {
+ print "Could not open $t: $!\n";
+ exit 1;
+ }
+
+ if (!open($file, '<', $f)) {
+ print "Could not open $f: $1\n";
+ exit 1;
+ }
+
+ while (my $row = <$file>) {
+ chomp $row;
+
+ if ($row =~ m/LMM_TAG\((.+)\)/) {
+ # yup...
+ print $ctags "$1\t$f\t/LMM_TAG($1)/;\"\td\n";
+ }
+ }
+ close($file);
+ close($ctags);
+}
+
+if ($#ARGV != 1) {
+ print "Usage:\n\tscripts/ctags-parse-lmm.pl tags C-file-to-parse\n";
+ exit 1;
+}
+
+parse($ARGV[0], $ARGV[1]);
+exit 0;
--
2.23.0
======================================================================

The next thing I did was

./scripts/ctags-parse-lmm.pl ./tags kernel/printk/dataring.c
./scripts/ctags-parse-lmm.pl ./tags kernel/printk/numlist.c
./scripts/ctags-parse-lmm.pl ./tags kernel/printk/ringbuffer.c

These 3 commands added the following entries to the tags file
(I'm using ctags and vim)

======================================================================
$ tail tags
fA kernel/printk/dataring.c /LMM_TAG(fA)/;" d
fB kernel/printk/dataring.c /LMM_TAG(fB)/;" d
fC kernel/printk/dataring.c /LMM_TAG(fC)/;" d
fE kernel/printk/dataring.c /LMM_TAG(fE)/;" d
fF kernel/printk/dataring.c /LMM_TAG(fF)/;" d
fG kernel/printk/dataring.c /LMM_TAG(fG)/;" d
fH kernel/printk/dataring.c /LMM_TAG(fH)/;" d
cB kernel/printk/numlist.c /LMM_TAG(cB)/;" d
cD kernel/printk/numlist.c /LMM_TAG(cD)/;" d
cA kernel/printk/numlist.c /LMM_TAG(cA)/;" d
======================================================================

So now when I perform LMM tag search or jump to a tag definition, vim
goes exactly to the line where the corresponding LMM_TAG was defined.

Example:

kernel/printk/ringbuffer.c

RELEASE from jA->cD->hA to jB
^
C-] // jump to tag under cursor

vim goes to kernel/printk/numlist.c

360 * LMM_TAG(cD)
^

Exactly where cD was defined.

Welcome to the future!

> Andrea suggested that the documentation should be within the code, which
> I think is a good idea. Even if it means we have more comments than
> code.

I agree that such documentation is handy. It, probably, would be even better
if we could use some tooling to make it easier to use.

-ss