![[BACK]](/icons/back.gif){kind=icon}
Up to [cvs.NetBSD.org] / src / share / man / man9
Request diff between arbitrary revisions
Keyword substitution: kv
Default branch: MAIN
Pull up following revision(s) (requested by rin in ticket #728): share/man/man9/atomic_loadstore.9: revision 1.8 atomic_loadstore(9): Clarify relation to __HAVE_ATOMIC64_OPS. PR kern/58340
atomic_loadstore(9): Clarify relation to __HAVE_ATOMIC64_OPS. PR kern/58340
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.
Pull up following revision(s) (requested by riastradh in ticket #1082): share/man/man9/atomic_loadstore.9: revision 1.6 share/man/man9/ipi.9: revision 1.5 lib/libc/atomic/membar_ops.3: revision 1.6 Spell out acronyms in title for clarity. Update membar_ops(3) man page with examples and relation to C11. Add exhortation to always always always document how membars come in pairs for synchronization between two CPUs when you use them. atomic_load/store_* appeared in NetBSD 9, not 10. Pullup preceded release of 9.0.
atomic_load/store_* appeared in NetBSD 9, not 10. Pullup preceded release of 9.0.
Merge changes from current as of 20200406
Pull up following revision(s) (requested by riastradh in ticket #508): distrib/sets/lists/comp/mi: revision 1.2294 share/man/man9/Makefile: revision 1.443 sys/sys/atomic.h: revision 1.18 share/man/man9/atomic_loadstore.9: revision 1.1 share/man/man9/atomic_loadstore.9: revision 1.2 New atomic load/store operations for the kernel. Guarantee no fusing and no tearing, and can optionally impose ordering relative to other memory operations. Unordered: - atomic_load_relaxed - atomic_store_relaxed Ordered: - atomic_load_acquire - atomic_load_consume - atomic_store_release These are intended to match C11 semantics, and can be defined in terms of the C11 atomic API when ready. Document relation to atomic_ops(3) and membar_ops(3).
file atomic_loadstore.9 was added on branch phil-wifi on 2020-04-08 14:07:22 +0000
file atomic_loadstore.9 was added on branch netbsd-9 on 2019-12-08 14:26:38 +0000
Simplify markup for inline code to plain .Li in most places. Use backslashes to protetect embedded spaces in inline code to prevent filling from stretching them. Use .Fn and .Fa to refer only to things this manual page documents, they are not to be used for "a function" or "a function argument" in general. In code examples drop the empty line at the beginning of a function (mandated by the KNF when there are no variables) - it doesn't help readability here.
Simplify macro usage.
Use .Sy, not .Em, for bold-faced WARNING of impending doom. Underline and, worse, italic aren't as good at catching the eye.
Document relation to atomic_ops(3) and membar_ops(3).
New atomic load/store operations for the kernel. Guarantee no fusing and no tearing, and can optionally impose ordering relative to other memory operations. Unordered: - atomic_load_relaxed - atomic_store_relaxed Ordered: - atomic_load_acquire - atomic_load_consume - atomic_store_release These are intended to match C11 semantics, and can be defined in terms of the C11 atomic API when ready.