linux-kernel - Re: Difference between atomic operations and memory barriers

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Message-ID: <4AE5F04E.3050908@nortel.com>
Date:	Mon, 26 Oct 2009 12:54:06 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	"Leonidas ." <leonidas137@...il.com>
CC:	Noah Watkins <noah@...hdesu.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Difference between atomic operations and memory barriers

On 10/26/2009 12:22 PM, Leonidas . wrote:
> On Mon, Oct 26, 2009 at 11:49 PM, Noah Watkins <noah@...hdesu.com> wrote:
>>> Trying to understand difference between atomic ops and memory barriers.
>>>
>> There is a bit of info in:
>>    Documentation/atomic_ops.txt

> Thanks for the pointer, there are no atomic operations for pointers, right?
> I guess barrier is what we need in that case.

You need barriers for the atomics too, at least in some cases.  From
that document:

*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***

For simple assignments, I'm fairly sure linux assumes that writes to
individual variables are atomic.  ie, if you're assigning to an int you
won't get half of the assignment completed while the other half isn't.

What the atomic ops buy you is the ability to do increment-and-test,
compare-and-exchange, etc. in an atomic fashion.

Barriers are needed to make sure that operations are visible on other
cpus in the correct order, and to ensure that the compiler doesn't
reorder the operations.

If you use the normal locking mechanisms (mutexes, spinlocks, etc.) the
barriers are implicit in the lock.

Chris
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