From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7663 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 19:19:17 +0200 Message-ID: <1431883157.4219.3.camel@inria.fr> References: <20150517045536.GA25046@brightrain.aerifal.cx> <1431845344.11628.1.camel@inria.fr> <20150517162221.GM17573@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="=-ZN1kPItprlZFTUHXXn2u" X-Trace: ger.gmane.org 1431883179 22686 80.91.229.3 (17 May 2015 17:19:39 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 17 May 2015 17:19:39 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7675-gllmg-musl=m.gmane.org@lists.openwall.com Sun May 17 19:19:34 2015 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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:7663 Archived-At: --=-ZN1kPItprlZFTUHXXn2u Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Am Sonntag, den 17.05.2015, 12:22 -0400 schrieb Rich Felker: > On Sun, May 17, 2015 at 08:49:04AM +0200, Jens Gustedt wrote: > > Hello, > >=20 > > 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 i= f > > > it can do so more efficiently. > >=20 > > I like the approach > >=20 > > > 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. > >=20 > > shouldn't this file then define or macros such as a_swap, too ? >=20 > Hm? I don't understand what you're asking. Ah, I missed the list of defines that are actually all hidden at the end of the file. I would have preference to put them in the beginning of the file. > > On quick inspection I found issues with the two 64 bit functions: > >=20 > > #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 > >=20 > > #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 > >=20 > > 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? >=20 > These are misnomers. They're only used/needed as atomic bit-set and > bit-clear. It would be nice to eliminate them completely, but malloc > is using them right now. It would be easy to put the above logic > directly in malloc and have the bitmasks be kept as a union of > uint64_t and int[], but that's mildly ugly too I think. >=20 > > 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. >=20 > The cast itself doesn't break aliasing rules. Only accessing the > memory as int does that. yes, sure, that's what I meant :) > The intent was that a_or would only access > the object via asm, so the C type rules would not apply -- that's how > things originally worked when we only had i386 and x86_64. But now > that a_or is a C wrapper for a_cas on many/most archs, we do have an > aliasing problem, I think. That makes me more eagar to remove these. yes, see my other answer about malloc > > 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. >=20 > I'm not so sure about that. I am quite sure. We recently had a discussion on that in the committee, and the outcome was basically what I was stating above. > See this question on SO, which has two > conflicting and both reasonable-sounding answers: >=20 > http://stackoverflow.com/questions/28654418/requirements-for-behavior-of-= pointer-to-volatile-pointing-to-non-volatile-object thanks for the pointer, I didn't knew about the text in the rationale. This could be an indication that the text as it is in the standard is a defect. > In any case, all objects used with atomics in musl are declared > volatile now, or that is the intent anyway. If I missed any please let > me know. I don't know of any, but I will look around a bit. 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 :: --=-ZN1kPItprlZFTUHXXn2u 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 iEYEABECAAYFAlVYzZUACgkQD9PoadrVN+JJcgCfSfWzhkM35pLISOysEDjCdIlD cVYAn3r7bE9UxGiviqE2hmlyePP55Dtx =zBRV -----END PGP SIGNATURE----- --=-ZN1kPItprlZFTUHXXn2u--