From: Jon Masters <jcm@jonmasters.org>
To: "Paul E. McKenney" <paulmck@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence
Date: Thu, 23 Apr 2020 23:36:19 -0400 [thread overview]
Message-ID: <f3cdec26-480e-d2e0-3e54-4b0536831fcd@jonmasters.org> (raw)
In-Reply-To: <1392672413-5114-2-git-send-email-paulmck () linux ! vnet ! ibm ! com>
Hi Paul,
On 2/17/14 4:26 PM, Paul E. McKenney wrote:
> The ACCESS_ONCE() primitive provides cache coherence, but the
> documentation does not clearly state this. This commit therefore upgrades
> the documentation.
<snip>
> + In short, ACCESS_ONCE() provides "cache coherence" for accesses from
> + multiple CPUs to a single variable.
(ACCESS_ONCE is now READ_ONCE/WRITE_ONCE but the above added the
original language around cache coherence)
I would argue that we might want to avoid describing it in this manner.
The hardware provides cache coherency in order to keep a single memory
location coherent between multiple observers. These kernel macros only
tell the compiler to perform the load once. They take advantage of the
properties of coherence in the presence of multiple observers.
Jon.
--
Computer Architect
next parent reply other threads:[~2020-04-24 6:41 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1392672413-5114-2-git-send-email-paulmck () linux ! vnet ! ibm ! com>
2020-04-24 3:36 ` Jon Masters [this message]
2020-04-27 22:59 ` [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence Paul E. McKenney
2014-02-17 21:26 [PATCH tip/core/rcu 0/6] Documentation changes for 3.15 Paul E. McKenney
2014-02-17 21:26 ` [PATCH tip/core/rcu 1/6] documentation: Document call_rcu() safety mechanisms and limitations Paul E. McKenney
2014-02-17 21:26 ` [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence Paul E. McKenney
2014-02-17 21:40 ` Josh Triplett
2014-02-17 22:52 ` Paul E. McKenney
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