CVS log for src/common/lib/libc/arch/riscv/atomic/membar_ops.S

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Default branch: MAIN

Revision 1.4 / (download) - annotate - [select for diffs], Sat Apr 9 23:32:52 2022 UTC (9 months, 3 weeks ago) by riastradh
Branch: MAIN
CVS Tags: netbsd-10-base, netbsd-10, HEAD
Changes since 1.3: +13 -1 lines
Diff to previous 1.3 (colored)

Introduce membar_acquire/release.  Deprecate membar_enter/exit.

The names membar_enter/exit were unclear, and the documentation of
membar_enter has disagreed with the implementations on sparc,
powerpc, and even x86(!) for the entire time it has been in NetBSD.

The terms `acquire' and `release' are ubiquitous in the literature
today, and have been adopted in the C and C++ standards to mean
load-before-load/store and load/store-before-store, respectively,
which are exactly the orderings required by acquiring and releasing a
mutex, as well as other useful applications like decrementing a
reference count and then freeing the underlying object if it went to
zero.

Originally I proposed changing one word in the documentation for
membar_enter to make it load-before-load/store instead of
store-before-load/store, i.e., to make it an acquire barrier.  I
proposed this on the grounds that

(a) all implementations guarantee load-before-load/store,
(b) some implementations fail to guarantee store-before-load/store,
and
(c) all uses in-tree assume load-before-load/store.

I verified parts (a) and (b) (except, for (a), powerpc didn't even
guarantee load-before-load/store -- isync isn't necessarily enough;
need lwsync in general -- but it _almost_ did, and it certainly didn't
guarantee store-before-load/store).

Part (c) might not be correct, however: under the mistaken assumption
that atomic-r/m/w then membar-w/rw is equivalent to atomic-r/m/w then
membar-r/rw, I only audited the cases of membar_enter that _aren't_
immediately after an atomic-r/m/w.  All of those cases assume
load-before-load/store.  But my assumption was wrong -- there are
cases of atomic-r/m/w then membar-w/rw that would be broken by
changing to atomic-r/m/w then membar-r/rw:

https://mail-index.netbsd.org/tech-kern/2022/03/29/msg028044.html

Furthermore, the name membar_enter has been adopted in other places
like OpenBSD where it actually does follow the documentation and
guarantee store-before-load/store, even if that order is not useful.
So the name membar_enter currently lives in a bad place where it
means either of two things -- r/rw or w/rw.

With this change, we deprecate membar_enter/exit, introduce
membar_acquire/release as better names for the useful pair (r/rw and
rw/w), and make sure the implementation of membar_enter guarantees
both what was documented _and_ what was implemented, making it an
alias for membar_sync.

While here, rework all of the membar_* definitions and aliases.  The
new logic follows a rule to make it easier to audit:

	membar_X is defined as an alias for membar_Y iff membar_X is
	guaranteed by membar_Y.

The `no stronger than' relation is (the transitive closure of):

- membar_consumer (r/r) is guaranteed by membar_acquire (r/rw)
- membar_producer (w/w) is guaranteed by membar_release (rw/w)
- membar_acquire (r/rw) is guaranteed by membar_sync (rw/rw)
- membar_release (rw/w) is guaranteed by membar_sync (rw/rw)

And, for the deprecated membars:

- membar_enter (whether r/rw, w/rw, or rw/rw) is guaranteed by
  membar_sync (rw/rw)
- membar_exit (rw/w) is guaranteed by membar_release (rw/w)

(membar_exit is identical to membar_release, but the name is
deprecated.)

Finally, while here, annotate some of the instructions with their
semantics.  For powerpc, leave an essay with citations on the
unfortunate but -- as far as I can tell -- necessary decision to use
lwsync, not isync, for membar_acquire and membar_consumer.

Also add membar(3) and atomic(3) man page links.

Revision 1.3 / (download) - annotate - [select for diffs], Sat Apr 9 22:53:53 2022 UTC (9 months, 3 weeks ago) by riastradh
Branch: MAIN
Changes since 1.2: +2 -2 lines
Diff to previous 1.2 (colored)

riscv/membar_ops: Upgrade membar_enter from W/RW to RW/RW.

This will be deprecated soon but let's avoid leaving rakes to trip on
with it arising from disagreement over the documentation (W/RW) and
implementation and usage (R/RW).

Revision 1.2 / (download) - annotate - [select for diffs], Wed Apr 6 22:47:56 2022 UTC (9 months, 3 weeks ago) by riastradh
Branch: MAIN
Changes since 1.1: +2 -2 lines
Diff to previous 1.1 (colored)

Nix trailing whitespace in files of membars, atomics, and lock stubs.

Will be touching many of these files soon for functional changes.

No functional change intended.

Revision 1.1.16.2 / (download) - annotate - [select for diffs], Tue Apr 21 19:37:45 2020 UTC (2 years, 9 months ago) by martin
Branch: phil-wifi
Changes since 1.1.16.1: +0 -0 lines
Diff to previous 1.1.16.1 (colored) to branchpoint 1.1 (colored) next main 1.2 (colored)

Ooops, restore accidently removed files from merge mishap

Revision 1.1.16.1, Tue Apr 21 18:41:20 2020 UTC (2 years, 9 months ago) by martin
Branch: phil-wifi
Changes since 1.1: +1 -1 lines
FILE REMOVED

Sync with HEAD

New files for Userland support of UCB RISC-V (both 32-bit and 64-bit)

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