linux-kernel - Re: linux-next: build warnings after merge of the access

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Date:	Thu, 26 Mar 2015 17:28:57 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Will Deacon <will.deacon@....com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: linux-next: build warnings after merge of the access_once tree

On Thu, Mar 26, 2015 at 09:15:21AM -0700, Linus Torvalds wrote:
> Notice how it is *not* about atomicitiy. The compiler can read the
> value in fifteen pieces, randomly mixing one bit or five. Nobody
> cares.

If you read Documentation/memory-barriers.txt you'll find that it very
much also is about reading it in one go.

"The ACCESS_ONCE() function can prevent any number of optimizations that,
while perfectly safe in single-threaded code, can be fatal in concurrent
code.  Here are some examples of these sorts of optimizations:

 ...

 (*) For aligned memory locations whose size allows them to be accessed
     with a single memory-reference instruction, prevents "load tearing"
     and "store tearing," ..."

There are many places in the kernel where we rely and use ACCESS_ONCE()
in order to 'guarantee' single loads. Paul is the expert here, but from
what I understand the compiler is not allowed to split loads for
volatile reads (assuming the load is both naturally aligned and of
machine word size).

And the size check in READ_ONCE() helps asserting this.
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