Re: [patch] ipvs: force read of atomic_t in while loop

From: Chris Snook
Date: Wed Aug 08 2007 - 17:14:50 EST


Heiko Carstens wrote:
On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote:
From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Date: Wed, 8 Aug 2007 11:33:00 +0200

Just saw this while grepping for atomic_reads in a while loops.
Maybe we should re-add the volatile to atomic_t. Not sure.
I think whatever the choice, it should be done consistently
on every architecture.

It's just asking for trouble if your arch does it differently from
every other.

Well..currently it's i386/x86_64 and s390 which have no volatile
in atomic_t. And yes, of course I agree it should be consistent
across all architectures. But it isn't.

Based on recent discussion, it's pretty clear that there's a lot of confusion about this. A lot of people (myself included, until I thought about it long and hard) will reasonably assume that calling atomic_read() will actually read the value from memory. Leaving out the volatile declaration seems like a pessimization to me. If you force people to use barrier() everywhere they're working with atomic_t, it will force re-reads of all the non-atomic data in use as well, which will cause more memory fetches of things that generally don't need barrier(). That and it's a bug waiting to happen.

Andi -- your thoughts on the matter?
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