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Re: [dm-devel] [PATCH 5/5] aio: Refactor aio_read_evt, use cmxchg(), fix bug



On Mon, Oct 08, 2012 at 11:39:20PM -0700, Kent Overstreet wrote:
> Bunch of cleanup

Ugh.  That's way too much noisy change for one patch with no
description.  Break it up into functional pieces and actually describe
them.

> events off the ringbuffer without racing with io_getevents().

Are you sure this is safe in the presence of wrapping indices?  It's
been a very long time since I've looked at this, but I could never
convince myself that it was safe.

What I'm worried about is cmpxchg()s caller sampling, say, and index of
0, having another task GO NUTS and wrap that index all the way around
back to 0, and then having that initial cmpchg() caller see the wrapped
0 index and think that's nothing's changed in the interim.

Is this a problem?

(I wish I could find the comment I wrote in a very old patch to tear out
the mapped ring entirely.. It was a great list of its design mistakes.)

- z


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