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Re: [Qemu-block] [PATCH v2] block/vdi: Add locking for parallel requests

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v2] block/vdi: Add locking for parallel requests
Date: Fri, 27 Feb 2015 13:27:20 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 2015-02-27 at 13:09, Max Reitz wrote:
On 2015-02-27 at 12:42, Paolo Bonzini wrote:

On 27/02/2015 15:05, Max Reitz wrote:
Concurrently modifying the bmap does not seem to be a good idea; this patch adds a lock for it. See https://bugs.launchpad.net/qemu/+bug/1422307 for what
can go wrong without.

Cc: qemu-stable <address@hidden>
Signed-off-by: Max Reitz <address@hidden>
- Make the mutex cover vdi_co_write() completely [Kevin]
- Add a TODO comment [Kevin]
I think I know what the bug is.  Suppose you have two concurrent writes
to a non-allocated block, one at 16K...32K (in bytes) and one at
32K...48K.  The first write is enlarged to contain zeros, the second is
not.  Then you have two writes in flight:

       0       zeros
       ...     zeros
       16K     data1
       ...     data1
       32K     zeros      data2
       ...     zeros      data2
       48K     zeros
       ...     zeros

And the contents of 32K...48K are undefined.  If the above diagnosis is
correct, I'm not even sure why Max's v1 patch worked...

Maybe that's an issue, too; but the test case I sent out does 1 MB requests (and it fails), so this shouldn't matter there.

Considering that test case didn't work for Stefan (Weil), and it fails in a pretty strange way for me (no output from the qemu-io command at all, and while most reads from raw were successful, all reads from vdi failed (the pattern verification failed), maybe that's something completely different.

Indeed, when I do sub-MB writes, I get sporadic errors which seem much more related to the original bug report, so it's probably the issue you found that's the real problem.

Also, my test case suddenly stopped reproducing the issue on my HDD and only does it on tmpfs. Weird.


An optimized fix could be to use a CoRwLock, then:

Yes, I'm actually already working on that.


- take it shared (read) around the write in the
"VDI_IS_ALLOCATED(bmap_entry)" path

- take it exclusive (write) around the write in the
"!VDI_IS_ALLOCATED(bmap_entry)" path


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