From: Peter Zijlstra <peterz@infradead•org>
To: Linus Torvalds <torvalds@linux-foundation•org>
Cc: Will Deacon <will.deacon@arm•com>,
Stephen Rothwell <sfr@canb•auug.org.au>,
Christian Borntraeger <borntraeger@de•ibm.com>,
Thomas Gleixner <tglx@linutronix•de>, Ingo Molnar <mingo@elte•hu>,
"H. Peter Anvin" <hpa@zytor•com>,
"linux-next@vger•kernel.org" <linux-next@vger•kernel.org>,
"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>,
Davidlohr Bueso <dave@stgolabs•net>,
Paul McKenney <paulmck@linux•vnet.ibm.com>
Subject: Re: linux-next: build warnings after merge of the access_once tree
Date: Thu, 26 Mar 2015 17:28:57 +0100 [thread overview]
Message-ID: <20150326162857.GB21418@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CA+55aFyLTc72-xTkjugVSMb3EQq=+__UcFe28agu086xf9d6ZA@mail.gmail.com>
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.
next prev parent reply other threads:[~2015-03-26 16:29 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-26 8:31 linux-next: build warnings after merge of the access_once tree Stephen Rothwell
2015-03-26 10:11 ` Christian Borntraeger
2015-03-26 10:34 ` Peter Zijlstra
2015-03-26 13:27 ` Will Deacon
2015-03-26 14:22 ` Peter Zijlstra
2015-03-26 14:41 ` Will Deacon
2015-03-26 14:51 ` Peter Zijlstra
2015-03-26 15:08 ` Will Deacon
2015-03-26 16:15 ` Linus Torvalds
2015-03-26 16:21 ` Linus Torvalds
2015-03-26 16:36 ` Peter Zijlstra
2015-03-26 16:44 ` Peter Zijlstra
2015-03-26 16:45 ` Peter Zijlstra
[not found] ` <CA+55aFw1WHJqSj+z-mJGY-kxrg_OsGp9jK9VBi+wB4zPgCkv_w@mail.gmail.com>
2015-03-26 17:07 ` Peter Zijlstra
2015-03-26 17:17 ` Will Deacon
2015-03-26 17:23 ` Christian Borntraeger
2015-03-26 19:42 ` Christian Borntraeger
2015-03-26 16:28 ` Peter Zijlstra [this message]
[not found] ` <CA+55aFzUPPSHakwbp-Y-SaXB+o1=V6rOknz7L3AYNXNPU1MSfg@mail.gmail.com>
2015-03-26 17:12 ` Paul E. McKenney
2015-03-26 17:24 ` Christian Borntraeger
2015-03-26 17:52 ` Linus Torvalds
2015-03-26 18:54 ` Christian Borntraeger
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