lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Tue, 18 Feb 2014 09:44:48 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Torvald Riegel <triegel@...hat.com> Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>, Will Deacon <will.deacon@....com>, Peter Zijlstra <peterz@...radead.org>, Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>, David Howells <dhowells@...hat.com>, "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "mingo@...nel.org" <mingo@...nel.org>, "gcc@....gnu.org" <gcc@....gnu.org> Subject: Re: [RFC][PATCH 0/5] arch: atomic rework On Tue, Feb 18, 2014 at 8:17 AM, Torvald Riegel <triegel@...hat.com> wrote: >> >> "Consume operation: no reads in the current thread dependent on the >> value currently loaded can be reordered before this load" > > I can't remember seeing that language in the standard (ie, C or C++). > Where is this from? That's just for googling for explanations. I do have some old standard draft, but that doesn't have any concise definitions anywhere that I could find. >> and it could make a compiler writer say that value speculation is >> still valid, if you do it like this (with "ptr" being the atomic >> variable): >> >> value = ptr->val; > > I assume the load from ptr has mo_consume ordering? Yes. >> into >> >> tmp = ptr; >> value = speculated.value; >> if (unlikely(tmp != &speculated)) >> value = tmp->value; >> >> which is still bogus. The load of "ptr" does happen before the load of >> "value = speculated->value" in the instruction stream, but it would >> still result in the CPU possibly moving the value read before the >> pointer read at least on ARM and power. > > And surprise, in the C/C++ model the load from ptr is sequenced-before > the load from speculated, but there's no ordering constraint on the > reads-from relation for the value load if you use mo_consume on the ptr > load. Thus, the transformed code has less ordering constraints than the > original code, and we arrive at the same outcome. Ok, good. > The standard is clear on what's required. I strongly suggest reading > the formalization of the memory model by Batty et al. Can you point to it? Because I can find a draft standard, and it sure as hell does *not* contain any clarity of the model. It has a *lot* of verbiage, but it's pretty much impossible to actually understand, even for somebody who really understands memory ordering. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists