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

[Paolo Bonzini]

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.

Paolo