linux-kernel - Re: [PATCH 1/2] FRV: Implement atomic64

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Date:	Thu, 2 Jul 2009 14:28:23 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Howells <dhowells@...hat.com>, mingo@...e.hu,
	akpm@...ux-foundation.org, paulus@...ba.org, arnd@...db.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] FRV: Implement atomic64_t

On Thu, 2 Jul 2009, Eric Dumazet wrote:
> 
> Using a fixed initial value (instead of __atomic64_read()) is even faster, 
> it apparently permits cpu to use an appropriate bus transaction.

Yeah, I guess it does a "read-for-write-ownership" and allows the thing to 
be done as a single cache transaction.

If we read it first, it will first get the cacheline for shared-read, and 
then the cmpxchg8b will need to turn it from shared to exclusive.

Of course, the _optimal_ situation would be if the cmpxchg8b didn't 
actually do the write at all when the value matches (and all cores could 
just keep it shared), but I guess that's not going to happen.

Too bad there is no pure 8-byte read op. Using MMX has too many downsides.

Btw, your numbers imply that for the atomic64_add_return(), we really 
would be much better off not reading the original value at all. Again, in 
that case, we really do want the "read-for-write-ownership" cache 
transaction, not a read.

		Linus
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