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Subject: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use
 bdrv_(p)write_sync for metadata writes"
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, stefanha@gmail.com, mjt@tls.msk.ru, qemu-devel@nongnu.org, hch@lst.de

  On 08/24/2010 04:35 PM, Anthony Liguori wrote:
>> It's about metadata writes.  If an operation changes metadata, we 
>> must sync it to disk before writing any data or other metadata which 
>> depends on it, regardless of any promises to the guest.
>
>
> Why?  If the metadata isn't sync, we loose the write.
>
> But that can happen anyway because we're not sync'ing the data
>
> We need to sync the metadata in the event of a guest initiated flush, 
> but we shouldn't need to for a normal write.

1. Allocate a cluster (increase refcount table)

2. Link cluster to L2 table

3. Second operation makes it to disk; first still in pagecache

4. Crash

5. Dangling pointer from L2 to freed cluster

-- 
error compiling committee.c: too many arguments to function