[pve-devel] [PATCH v2 storage] lvm plugin: fix locking for rollback when using CLI

[Fiona Ebner <f.ebner@proxmox.com>]

Doing a rollback via CLI on an LVM storage with 'saferemove' and
'snapshot-as-volume-chain' would run into a locking issue, because
the forked zero-out worker would try to acquire the lock while the
main CLI task is still inside the locked section for
volume_snapshot_rollback_locked(). The same issue does not happen when
the rollback is done via UI. The reason for this can be found in the
note regarding fork_worker():

> we simulate running in foreground if ($self->{type} eq 'cli')

So the worker will be awaited synchronously in CLI context, resulting
in the deadlock, while via API/UI, the main task would move on and
release the lock allowing the zero-out worker to acquire it.

Avoid doing fork_cleanup_worker() inside the locked section to avoid
the issue.

Fixes: 8eabcc7 ("lvm plugin: snapshot-as-volume-chain: use locking for snapshot operations")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v2:
* Better explanation in commit message.
* Also spawn cleanup worker in error scenario like before the patch.

 src/PVE/Storage/LVMPlugin.pm | 43 ++++++++++++++++++++++--------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/src/PVE/Storage/LVMPlugin.pm b/src/PVE/Storage/LVMPlugin.pm
index 97f7bf4..102cf22 100644
--- a/src/PVE/Storage/LVMPlugin.pm
+++ b/src/PVE/Storage/LVMPlugin.pm
@@ -1117,23 +1117,17 @@ sub volume_rollback_is_possible {
 }
 
 my sub volume_snapshot_rollback_locked {
-    my ($class, $scfg, $storeid, $volname, $snap) = @_;
+    my ($class, $scfg, $storeid, $volname, $snap, $cleanup_worker) = @_;
 
     my $format = ($class->parse_volname($volname))[6];
 
     die "can't rollback snapshot for '$format' volume\n" if $format ne 'qcow2';
 
-    my $cleanup_worker = eval { free_snap_image($class, $storeid, $scfg, $volname, 'current'); };
+    $cleanup_worker->$* = eval { free_snap_image($class, $storeid, $scfg, $volname, 'current'); };
     die "error deleting snapshot $snap $@\n" if $@;
 
     eval { alloc_snap_image($class, $storeid, $scfg, $volname, $snap) };
-    my $alloc_err = $@;
-
-    fork_cleanup_worker($cleanup_worker);
-
-    if ($alloc_err) {
-        die "can't allocate new volume $volname: $alloc_err\n";
-    }
+    die "can't allocate new volume $volname: $@\n" if $@;
 
     return undef;
 }
@@ -1141,14 +1135,29 @@ my sub volume_snapshot_rollback_locked {
 sub volume_snapshot_rollback {
     my ($class, $scfg, $storeid, $volname, $snap) = @_;
 
-    return $class->cluster_lock_storage(
-        $storeid,
-        $scfg->{shared},
-        undef,
-        sub {
-            return volume_snapshot_rollback_locked($class, $scfg, $storeid, $volname, $snap);
-        },
-    );
+    my $cleanup_worker;
+
+    eval {
+        $class->cluster_lock_storage(
+            $storeid,
+            $scfg->{shared},
+            undef,
+            sub {
+                volume_snapshot_rollback_locked(
+                    $class, $scfg, $storeid, $volname, $snap, \$cleanup_worker,
+                );
+            },
+        );
+    };
+    my $err = $@;
+
+    # Spawn outside of the locked section, because with 'saferemove', the cleanup worker also needs
+    # to obtain the lock, and in CLI context, it will be awaited synchronously, see fork_worker().
+    fork_cleanup_worker($cleanup_worker);
+
+    die $err if $err;
+
+    return;
 }
 
 sub volume_snapshot_delete {
-- 
2.47.3



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