Re: [Qemu-devel] [PATCH] linux-aio: fix submit aio as a batch

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jul 08, 2014 at 11:45:10PM +0800, Ming Lei wrote:
> In the enqueue path, we can't complete request, otherwise
> "Co-routine re-entered recursively" may be caused, so this
> patch fixes the issue with below ideas:

Thi  probably happens when the caller is in coroutine context and its
completion function invokes qemu_coroutine_enter() on itself.  The
solution is to invoke completions from a BH (other places in the block
layer do this too).

> 	- for -EAGAIN, retry the submission in an introduced event handler

I agree with Paolo that a BH is appropriate.

> 	- for part of completion, just update the io queue, since it is
> 	moving on after all

If we do this then we need to guarantee that io_submit() will be called
at some point soon.  Otherwise requests could get stuck if the guest
doesn't submit any more I/O requests to push the queue.

Please split this into separate patches.  You're trying to do too much.

Overall, I would prefer it if we avoid the extra complexity of deferring
io_submit() on EAGAIN and partial submission.  Do you understand why the
kernel is producing this behavior?  Can we set the right capacity in
io_setup() so it doesn't happen?

> +        if (enqueue)
> +            return ret;

Please set up a git hook to run checkpatch.pl.  It will alert you when
you violate QEMU coding style:
http://blog.vmsplice.net/2011/03/how-to-automatically-run-checkpatchpl.html

I already mentioned coding style in previous patches, using a git hook
will avoid it happening again.

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