Re: [Qemu-devel] [PATCH RFC 0/9] block: Rewrite block drain begin/end

[Kevin Wolf]

Am 29.11.2017 um 15:49 hat Fam Zheng geschrieben:
> While we look at the fixes for 2.11, I briefly prototyped this series
> to see if it makes sense as a simplification of the drain API for
> 2.12.
> 
> The idea is to let AioContext manage quiesce callbacks, then the block
> layer only needs to do the in-flight request waiting. This lets us get
> rid of the callback recursion (both up and down).

So essentially you don't drain individual nodes any more, but whole
AioContexts. I have a feeeling that this would be a step in the wrong
direction.

Not only would it completely bypass the path I/O requests take and
potentially drain a lot more than is actually necessary, but it also
requires that all nodes that are connected in a tree are in the same
AioContext.

Paolo can say more on this, but my understanding was that the long-term
plan is to make the block layer fully thread safe so that any thread
could call things on any node. I remember Paolo saying that AioContext
was even fully going away in the future. I don't see how this is
compatible with your approach.

And finally, I don't really think that the recursion is even a problem.
The problem is with graph changes made by callbacks that drain allows to
run. With your changes, it might be a bit easier to avoid that
bdrv_drain() itself gets into trouble due to graph changes, but this
doesn't solve the problem for any (possibly indirect) callers of
bdrv_drain().

Kevin