md/raid5: avoid livelock caused by non-aligned writes.
commit a40687ff upstream. If a stripe in a raid6 array received a write to each data block while the array is degraded, and if any of these writes to a missing device are not page-aligned, then a live-lock happens. In this case the P and Q blocks need to be read so that the part of the missing block which is *not* being updated by the write can be constructed. Due to a logic error, these blocks are not loaded, so the update cannot proceed and the stripe is 'handled' repeatedly in an infinite loop. This bug is unlikely as most writes are page aligned. However as it can lead to a livelock it is suitable for -stable. It was introduced in 3.16. Fixed: 67f45548Signed-off-by: NeilBrown <neilb@suse.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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