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2022-04-09Introduce membar_acquire/release. Deprecate membar_enter/exit.riastradh
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.
2022-04-06Nix trailing whitespace in files of membars, atomics, and lock stubs.riastradh
Will be touching many of these files soon for functional changes. No functional change intended.
2015-01-08Introduce membar_datadep_consumer.riastradh
Discussed briefly on tech-kern without objection: https://mail-index.netbsd.org/tech-kern/2014/11/20/msg018054.html https://mail-index.netbsd.org/tech-kern/2015/01/07/msg018326.html
2009-01-04allow inclusion of atomic ops in librumppooka
2008-09-29Allow atomic ops to be built as part of libpthread.ad
2008-05-25enable profiling of assembly functions.chs
2008-04-28Remove clause 3 and 4 from TNF licensesmartin
2008-02-16For all files under src/common, as well as related man pages underapb
src/lib/libc, or related headers under src/sys/sys: change u_intNN_t to uintNN_t.
2008-02-12Always enable memory barriers in userspace, otherwise libpthread won'tad
work correctly on MP systems.
2008-02-12Fix pthread problem. (export membar* to userland)matt
2008-02-11Only build atomic ops for libkern/libc.ad
2008-02-10Add atomic_cas_foo_ni().ad
2007-11-28Pull in alpha atomic ops from the thorpej-atomic branch.ad
2005-12-27make alpha kernel buildable again.yamt
don't add __ prefix to bswap{16,32} if defined(_KERNEL) || defined(_STANDALONE).
2005-12-21Only build this if GPROF && !_STANDALONEchristos
2005-12-20Merge libkern + libc common files. As requested by core.christos