linux-kernel - Re: [tip:locking/urgent] compiler, atomics: Provide READ_ONCE

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Message-ID: <20151014163423.GI3816@twins.programming.kicks-ass.net>
Date:	Wed, 14 Oct 2015 18:34:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	tip-bot for Andrey Ryabinin <tipbot@...or.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	kasan-dev <kasan-dev@...glegroups.com>,
	Ingo Molnar <mingo@...nel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Wolfram Gloger <wmglo@...t.med.uni-muenchen.de>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexander Potapenko <glider@...gle.com>
Subject: Re: [tip:locking/urgent] compiler, atomics: Provide
 READ_ONCE_NOCHECK ()

On Wed, Oct 14, 2015 at 09:23:33AM -0700, Andy Lutomirski wrote:
> On Wed, Oct 14, 2015 at 9:20 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Oct 14, 2015 at 06:18:58PM +0200, Dmitry Vyukov wrote:
> >>
> >> Well, if another thread writes it byte-by-byte, it pretty much does
> >> not matter how you read it.
> >> Note that I said "at least one access is not atomic". If both are
> >> atomic, then this is, of course, legal. And KTSAN considers
> >> READ/WRITE_ONCE as atomic operations.
> >
> > OK, then I'm confused on what exactly the annotation does, but less
> > worried.
> 
> The annotation says "hey, KASAN (etc), don't worry if you think that
> the memory being accessed is out of bounds".  Presumably KTSAN is okay
> with the operation because it's atomic, but KASAN dislikes it because
> it's accessing memory that is out of bounds from the perspective of a
> C program.

There's going to be more of that..

> I'd still rather find a way to just delete get_wchan, but whatever.

:-)
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