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Date: Fri, 3 Mar 2017 15:57:17 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
        kasan-dev <kasan-dev@googlegroups.com>,
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Subject: Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE
Message-ID: <20170303145717.GJ6536@twins.programming.kicks-ass.net> (sfid-20170303_182900_994436_B691737F)
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On Fri, Mar 03, 2017 at 03:49:38PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 03, 2017 at 09:26:50AM +0100, Christian Borntraeger wrote:
> > Right. The main purpose is to read/write _ONCE_. You can assume a somewhat
> > atomic access for sizes <= word size. And there are certainly places that
> > rely on that. But the *ONCE thing is mostly used for things where we used
> > barrier() 10 years ago.
> 
> A lot of code relies on READ/WRITE_ONCE() to generate single
> instructions for naturally aligned machined word sized loads/stores
> (something GCC used to guarantee, but does no longer IIRC).
> 
> So much so that I would say its a bug if READ/WRITE_ONCE() doesn't
> generate a single instruction under those conditions.
> 
> However, every time I've tried to introduce stricter
> semantics/primitives to verify things Linus hated it.

See here for the last attempt:

  https://marc.info/?l=linux-virtualization&m=148007765918101&w=2