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Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption
Date: Wed, 27 Jun 2012 08:48:17 +0100

On Wed, Jun 27, 2012 at 4:05 AM, Peter Crosthwaite
<address@hidden> wrote:
> I still think this thread points out a major flaw in block+coroutines,
> regardless of the fact im using it from a machine model. This bug is
> going to happen in any coroutine code that touches the block layer
> (E.G. what happens if the next developer wants to implement a device
> using coroutines?). Yes, without my full series there is no bug today,
> but im just trying to save the next developer who decides to use
> corourites (whether that be in tree or out of tree) the potentially
> several hours of debugging around "why did my coroutine get yielded
> randomly". That and of course minimisation of my own mainline diff.

The if (qemu_is_coroutine()) "fastpath" taken by the block layer today
hopefully won't be around forever.  It's really a shortcut that allows
code originally written with synchronous I/O in mind to work
unmodified in a coroutine.

Really we should get rid of bdrv_read() and friends so that all
callers use either bdrv_aio_*() or bdrv_co_*().  Then all functions
that yield will be marked coroutine_fn.  Then you know for sure that
the function may yield and you cannot rely on it not yielding.

I'd like to see your code though because I still don't understand why
it relies on the exact yield behavior.  Have you pushed it to a public
git repo?


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