From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7652 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: Deduplicating atomics written in terms of CAS Date: Sun, 17 May 2015 08:49:04 +0200 Message-ID: <1431845344.11628.1.camel@inria.fr> References: <20150517045536.GA25046@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; boundary="=-LUtMt8FZ9IRhXdSpNynO" X-Trace: ger.gmane.org 1431845360 19477 80.91.229.3 (17 May 2015 06:49:20 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 17 May 2015 06:49:20 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7664-gllmg-musl=m.gmane.org@lists.openwall.com Sun May 17 08:49:20 2015 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from 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Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.12.9-1+b1 Xref: news.gmane.org gmane.linux.lib.musl.general:7652 Archived-At: --=-LUtMt8FZ9IRhXdSpNynO Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hello, Am Sonntag, den 17.05.2015, 00:55 -0400 schrieb Rich Felker: > Lots of archs define most or all of their atomics except a_cas in > terms of a_cas. The attached atomic.h is a proposed replacement for > arch-specific atomic.h that would live in src/internal. The arch > atomic.h files would be replaced with atomic_arch.h, which could opt > to define nothing but a_cas, or could define more primitives itself if > it can do so more efficiently. I like the approach > The second attachment, atomic_generic.h, is an implementation of the > atomics (and other non-atomic ops we've traditionally had in atomic.h) > using GNU C builtins. This file can be used as-is for any new archs > that satisfy the following conditions: > > - They're not supported by compilers too old to have the __sync_* > builtins. >=20 > - They don't need runtime switching/detection of atomic > implementations. >=20 > - GCC doesn't generate pathologically bad code for the builtins. shouldn't this file then define or macros such as a_swap, too ? On quick inspection I found issues with the two 64 bit functions: #ifndef a_and_64 static inline void a_and_64(volatile uint64_t *p, uint64_t v) { union { uint64_t v; uint32_t r[2]; } u =3D { v }; if (u.r[0]+1) a_and((int *)p, u.r[0]); if (u.r[1]+1) a_and((int *)p+1, u.r[1]); } #endif #ifndef a_or_64 static inline void a_or_64(volatile uint64_t *p, uint64_t v) { union { uint64_t v; uint32_t r[2]; } u =3D { v }; if (u.r[0]) a_or((int *)p, u.r[0]); if (u.r[1]) a_or((int *)p+1, u.r[1]); } #endif First I don't get it how we can expect these to be be atomic. It looks to me that the two 32 bit words can be updated with quite a laps of time between them if the thread is delayed. I didn't check this, do we really need 64bit atomics? Then, the mix of uint32_t and int is unfortunate. This code is in header files and thus visible to all compilation units, especially user code that might use any optimization option that the compiler offers. The cast to int* breaks aliasing rules, so compilers that are used with aggressive optimization may produce wrong executables, in pretending that *p didn't change. I only recently learned that even cast to volatile doesn't help in cases where the original object to which p points is not declared volatile. The C standard states that only volatile *declared* objects are subject to the rules of volatile. Accessing through a volatile pointer doesn't help. Jens --=20 :: INRIA Nancy Grand Est ::: Camus ::::::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: --=-LUtMt8FZ9IRhXdSpNynO Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iEYEABECAAYFAlVYOeAACgkQD9PoadrVN+K3iQCfTOcgIsYFX7GDIhZAFvzkxDLN 9I8AnR3JlPXsGx5FO/a/FeYdeoYUtvNw =bFE6 -----END PGP SIGNATURE----- --=-LUtMt8FZ9IRhXdSpNynO--