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

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Message-Id: <a63e07dd8c48d22132c1292415a63c78@kernel.crashing.org>
Date:	Thu, 16 Aug 2007 21:39:18 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Christoph Lameter <clameter@....com>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Satyam Sharma <satyam@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org, ak@...e.de, cfriesen@...tel.com,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, zlynx@....org,
	schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

>> I can't speak for this particular case, but there could be similar 
>> code
>> examples elsewhere, where we do the atomic ops on an atomic_t object
>> inside a higher-level locking scheme that would take care of the kind 
>> of
>> problem you're referring to here. It would be useful for such or 
>> similar
>> code if the compiler kept the value of that atomic object in a 
>> register.
>
> If there is a higher-level locking scheme then there is no point to
> using atomic_t variables.  Atomic_t is specifically for the situation
> where multiple CPUs are updating a variable without locking.

And don't forget about the case where it is an I/O device that is
updating the memory (in buffer descriptors or similar).  The driver
needs to do a "volatile" atomic read to get at the most recent version
of that data, which can be important for optimising latency (or 
throughput
even).  There is no other way the kernel can get that info -- doing an
MMIO read is way way too expensive.


Segher

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