linux-kernel - Re: LDD3 pitfalls (was Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures)

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Message-ID: <46C996E5.90503@redhat.com>
Date:	Mon, 20 Aug 2007 09:28:05 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
CC:	Jonathan Corbet <corbet@....net>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	paulmck@...ux.vnet.ibm.com,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Christoph Lameter <clameter@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	ak@...e.de, heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org
Subject: Re: LDD3 pitfalls (was Re: [PATCH 0/24] make atomic_read() behave
 consistently across all architectures)

Stefan Richter wrote:
> Nick Piggin wrote:
>> Stefan Richter wrote:
>>> Nick Piggin wrote:
>>>
>>>> I don't know why people would assume volatile of atomics. AFAIK, most
>>>> of the documentation is pretty clear that all the atomic stuff can be
>>>> reordered etc. except for those that modify and return a value.
>>>
>>> Which documentation is there?
>> Documentation/atomic_ops.txt
>>
>>
>>> For driver authors, there is LDD3.  It doesn't specifically cover
>>> effects of optimization on accesses to atomic_t.
>>>
>>> For architecture port authors, there is Documentation/atomic_ops.txt.
>>> Driver authors also can learn something from that document, as it
>>> indirectly documents the atomic_t and bitops APIs.
>>>
>> "Semantics and Behavior of Atomic and Bitmask Operations" is
>> pretty direct :)
>>
>> Sure, it says that it's for arch maintainers, but there is no
>> reason why users can't make use of it.
> 
> 
> Note, LDD3 page 238 says:  "It is worth noting that most of the other
> kernel primitives dealing with synchronization, such as spinlock and
> atomic_t operations, also function as memory barriers."
> 
> I don't know about Linux 2.6.10 against which LDD3 was written, but
> currently only _some_ atomic_t operations function as memory barriers.
> 
> Besides, judging from some posts in this thread, saying that atomic_t
> operations dealt with synchronization may not be entirely precise.

atomic_t is often used as the basis for implementing more sophisticated 
synchronization mechanisms, such as rwlocks.  Whether or not they are designed 
for that purpose, the atomic_* operations are de facto synchronization primitives.

	-- Chris
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