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Re: [Qemu-devel] [PATCH] block: don't set the same context

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] block: don't set the same context
Date: Fri, 15 Feb 2019 06:29:50 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 2/15/19 3:19 AM, Denis Plotnikov wrote:
> Adds a fast path on aio context setting preventing
> unnecessary context setting routine.
> Also, it prevents issues with cyclic walk of child
> bds-es appeared because of registring aio walking


> notifiers:

> This can happen because of "new" context attachment to VM disk bds.
> When attaching a new context the corresponding aio context handler is
> called for each of aio_notifiers registered on the VM disk bds context.
> Among those handlers, there is the block_job_attached_aio_context handler
> which sets a new aio context for the block job bds. When doing so,
> the old context is detached from all the block job bds children and one of
> them is the VM disk bds, serving as backing store for the blockjob bds,
> although the VM disk bds is actually the initializer of that process.
> Since the VM disk bds is protected with walking_aio_notifiers flag
> from double processing in recursive calls, the assert fires.
> Signed-off-by: Denis Plotnikov <address@hidden>
> ---
>  block.c | 4 ++++
>  1 file changed, 4 insertions(+)

Naive question - do we have to always call
attach_aio_context/detach_aio_context in pairs?  If so, will
short-circuiting a nested attach cause reference count problems when we
come to the matching detach calls?

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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