linux-kernel - Documentation/memory-barriers.txt: How can READ_ONCE() and WRITE

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Date:	Sat, 27 Feb 2016 00:14:21 +0300
From:	Sergey Fedorov <serge.fdrv@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Documentation/memory-barriers.txt: How can READ_ONCE() and
 WRITE_ONCE() provide cache coherence?

Hi,

I just can't understand how this kind of compiler barrier macros may 
provide any form of cache coherence. Sure, such kind of compiler barrier 
is necessary to "reliably" access a variable from multiple CPUs. But why 
it is stated that these macros *provide* cache coherence?

 From Documentation/memory-barriers.txt:
> The READ_ONCE() and WRITE_ONCE() functions 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:
>
>  (*) The compiler is within its rights to reorder loads and stores
>      to the same variable, and in some cases, the CPU is within its
>      rights to reorder loads to the same variable.  This means that
>      the following code:
>
>     a[0] = x;
>     a[1] = x;
>
>      Might result in an older value of x stored in a[1] than in a[0].
>      Prevent both the compiler and the CPU from doing this as follows:
>
>     a[0] = READ_ONCE(x);
>     a[1] = READ_ONCE(x);
>
>      In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for
>      accesses from multiple CPUs to a single variable.

Thanks,
Sergey