Am 01.03.2012 17:22, schrieb Stefan Hajnoczi: > On Thu, Mar 01, 2012 at 05:10:57PM +0100, Kevin Wolf wrote: >> Am 27.02.2012 14:16, schrieb Stefan Hajnoczi: >>> The L2 table cache reduces QED metadata reads that would be required >>> when translating LBAs to offsets into the image file. Since requests >>> execute in parallel it is possible to share an L2 table between multiple >>> requests. >>> >>> There is a potential data corruption issue when an in-use L2 table is >>> evicted from the cache because the following situation occurs: >>> >>> 1. An allocating write performs an update to L2 table "A". >>> >>> 2. Another request needs L2 table "B" and causes table "A" to be >>> evicted. >>> >>> 3. A new read request needs L2 table "A" but it is not cached. >>> >>> As a result the L2 update from #1 can overlap with the L2 fetch from #3. >>> We must avoid doing overlapping I/O requests here since the worst case >>> outcome is that the L2 fetch completes before the L2 update and yields >>> stale data. In that case we would effectively discard the L2 update and >>> lose data clusters! >>> >>> Thanks to Benoît Canet <benoit.ca...@gmail.com> for extensive testing >>> and debugging which lead to discovery of this bug. >>> >>> Reported-by: Benoît Canet <benoit.ca...@gmail.com> >>> Signed-off-by: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> >> >> Thanks, applied to the block branch. >> >> How about a qemu-iotests case? > > The test case is not ready yet. I started writing one but it is racy > because I haven't introduced a way of controlling AIO issue/complete for > tests. My next step is to add that.
Will it be specific to image formats using AIO then or is it generic enough that coroutine-based drivers work with it as well? Kevin
