Re: [Qemu-block] [Qemu-devel] [PATCH 4/4] block-backend: Queue requests while drained

[Eric Blake]

On 7/25/19 11:27 AM, Kevin Wolf wrote:
> This fixes device like IDE that can still start new requests from I/O
> handlers in the CPU thread while the block backend is drained.
> 
> The basic assumption is that in a drain section, no new requests should
> be allowed through a BlockBackend (blk_drained_begin/end don't exist,
> we get drain sections only on the node level). However, there are two
> special cases where requests should not be queued:
> 
> 1. Block jobs: We already make sure that block jobs are paused in a
>    drain section, so they won't start new requests. However, if the
>    drain_begin is called on the job's BlockBackend first, it can happen
>    that we deadlock because the job stays busy until it reaches a pause
>    point - which it can't if it's requests aren't processed any more.

its (remember, "it's" is only okay if "it is" works as well)

> 
>    The proper solution here would be to make all requests through the
>    job's filter node instead of using a BlockBackend. For now, just
>    disabling request queuin on the job BlockBackend is simpler.

queuing

> 
> 2. In test cases where making requests through bdrv_* would be
>    cumbersome because we'd need a BdrvChild. As we already got the
>    functionality to disable request queuing from 1., use it in tests,
>    too, for convenience.
> 
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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