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Re: [Qemu-devel] [PATCH v3] migration/block:limit the time used for bloc

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v3] migration/block:limit the time used for block migration
Date: Sat, 8 Apr 2017 18:09:16 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 07/04/2017 19:33, Stefan Hajnoczi wrote:
> The migration thread is holding the QEMU global mutex, the AioContext,
> and the qcow2 s->lock while the L2 table is read from disk.
> The QEMU global mutex is needed for block layer operations that touch
> the global drives list.  bdrv_is_allocated() can be called without the
> global mutex.

Hi Stefan,

only virtio-blk and virtio-scsi take the AioContext lock (because they
support dataplane).  For block migration to work with devices such as
IDE, it needs to take the iothread lock too.  I think there's a comment
about this in migration/block.c.

However, this will hopefully be fixed in 2.10 by making the block layer
thread safe.


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