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Date: Fri, 10 Nov 2017 23:31:03 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH for-2.11] block: Keep strong reference when
 draining all BDS
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To: Max Reitz <mreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Fri, 11/10 16:23, Max Reitz wrote:
> But bdrv_unref() is safe only in the main loop.  Without having checked,
> I'm not sure whether all callers of bdrv_next() are running in the main
> loop.

They must be. The reasoning is simple:

1) one needs to acquire the ctx of all the BDSes for safe access;
2) only main loop can acquire any BDS' ctx;

So there is no way bdrv_next can work in an IOThread.

Fam