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Re: [Qemu-devel] [PATCH v2 09/13] block: Drain requests before swapping


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2 09/13] block: Drain requests before swapping nodes in bdrv_swap()
Date: Fri, 12 Jun 2015 16:16:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 10.06.2015 15:47, Kevin Wolf wrote:
bdrv_swap() requires that there are no requests in flight on either of
the two devices. The request coroutine would work on the wrong
BlockDriverState object (with bs->opaque even being interpreted as a
different type potentially) and all sorts of bad things would result
from this.

The currently existing callers mostly ensure that there is no I/O
pending on nodes that are swapped. In detail, this is:

1. Live snapshots. This goes through qmp_transaction(), which calls
    bdrv_drain_all() before doing anything. The command is executed
    synchronously, so no new I/O can be issued concurrently.

2. snapshot=on in bdrv_open(). We're in the middle of opening the image
    (both the original image and its temporary overlay), so there can't
    be any I/O in flight yet.

3. Mirroring. bdrv_drain() is already used on the source device so that
    the mirror doesn't miss anything. However, the main loop runs between
    that and the bdrv_swap() (which is actually a bug, being addressed in
    another series), so there is a small window in which new I/O might be
    issued that would be in flight during bdrv_swap().

It is safer to just drain the request queue of both devices in
bdrv_swap() instead of relying on callers to do the right thing.

Signed-off-by: Kevin Wolf <address@hidden>
---
  block.c | 6 ++++++
  1 file changed, 6 insertions(+)

Reviewed-by: Max Reitz <address@hidden>



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