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Re: [Qemu-block] [PATCH v12 02/19] block: Pause all jobs during bdrv_reo

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH v12 02/19] block: Pause all jobs during bdrv_reopen_multiple()
Date: Wed, 26 Oct 2016 16:36:12 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 26.10.2016 um 12:29 hat Alberto Garcia geschrieben:
> When a BlockDriverState is about to be reopened it can trigger certain
> operations that need to write to disk. During this process a different
> block job can be woken up. If that block job completes and also needs
> to call bdrv_reopen() it can happen that it needs to do it on the same
> BlockDriverState that is still in the process of being reopened.
> This can have fatal consequences, like in this example:
>   1) Block job A starts and sleeps after a while.
>   2) Block job B starts and tries to reopen node1 (a qcow2 file).
>   3) Reopening node1 means flushing and replacing its qcow2 cache.
>   4) While the qcow2 cache is being flushed, job A wakes up.
>   5) Job A completes and reopens node1, replacing its cache.
>   6) Job B resumes, but the cache that was being flushed no longer
>      exists.
> This patch splits the bdrv_drain_all() call to keep all block jobs
> paused during bdrv_reopen_multiple(), so that step 4 can never happen
> and the operation is safe.
> Note that this scenario can only happen if both bdrv_reopen() calls
> are made by block jobs on the same backing chain. Otherwise there's no
> chance that the same BlockDriverState appears in both reopen queues.
> Signed-off-by: Alberto Garcia <address@hidden>
> Reviewed-by: Paolo Bonzini <address@hidden>

Reviewed-by: Kevin Wolf <address@hidden>

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