Commits
Joe Thornber committed 2a0fbffb1e5
dm thin: fix a race condition between discarding and provisioning a block The discard passdown was being issued after the block was unmapped, which meant the block could be reprovisioned whilst the passdown discard was still in flight. We can only identify unshared blocks (safe to do a passdown a discard to) once they're unmapped and their ref count hits zero. Block ref counts are now used to guard against concurrent allocation of these blocks that are being discarded. So now we unmap the block, issue passdown discards, and the immediately increment ref counts for regions that have been discarded via passed down (this is safe because allocation occurs within the same thread). We then decrement ref counts once the passdown discard IO is complete -- signaling these blocks may now be allocated. This fixes the potential for corruption that was reported here: https://www.redhat.com/archives/dm-devel/2016-June/msg00311.html Reported-by: Dennis Yang <dennisyang@qnap.com> Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>