[Devel] [PATCH hci-8.1 1/2] dm-ploop: fix flip_upper_deltas returning with two md_locks held

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

ploop_check_delta_before_flip() takes the md_lock of a live BAT page
and of the matching page of the lower delta before it compares them.
When it finds a cluster both deltas map, it jumps to unmap past both
unlocks: the cmd returns -EEXIST with the locks held and interrupts
disabled, after freeing the page of the lower delta with its lock still
taken. dmsetup comes back to userspace in atomic context, and the next
pio touching that BAT page spins on its md_lock for good, a hard
lockup.

A flip_upper_deltas message, with both deltas opened read-write, on a
chain where the top delta overwrote a cluster of the lower one gets
there. The ploop library does not: its reverse merge clears those
entries in the lower delta before it flips, and a device it mounted has
the lower delta read-only, which the flip refuses before the check.

Leave the scan only after both locks are dropped.

Fixes: 0d4b4d0a20a9 ("dm-ploop: introduce per-md page locking")
https://virtuozzo.atlassian.net/browse/VSTOR-153445
Feature: dm-ploop: ploop target driver
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 drivers/md/dm-ploop-cmd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-ploop-cmd.c b/drivers/md/dm-ploop-cmd.c
index 45fa4fe7a45a..d6abbdc3c7e3 100644
--- a/drivers/md/dm-ploop-cmd.c
+++ b/drivers/md/dm-ploop-cmd.c
@@ -1146,11 +1146,13 @@ static int ploop_check_delta_before_flip(struct ploop *ploop, struct file *file)
 			    d_bat_entries[i] != BAT_ENTRY_NONE) {
 				ret = -EEXIST;
 				stop = true;
-				goto unmap;
+				break;
 			}
 		}
 		spin_unlock(&d_md->md_lock);
 		spin_unlock_irq(&md->md_lock);
+		if (stop)
+			break;
 
 		clu = ploop_page_clu_idx_to_bat_clu(md->id, i);
 		if (clu == nr_be - 1) {
-- 
2.53.0

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