4.20-stable review patch. If anyone has any objections, please let me know.
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From: Filipe Manana fdmanana@suse.com
commit 9a6f209e36500efac51528132a3e3083586eda5f upstream.
If the quota enable and snapshot creation ioctls are called concurrently we can get into a deadlock where the task enabling quotas will deadlock on the fs_info->qgroup_ioctl_lock mutex because it attempts to lock it twice, or the task creating a snapshot tries to commit the transaction while the task enabling quota waits for the former task to commit the transaction while holding the mutex. The following time diagrams show how both cases happen.
First scenario:
CPU 0 CPU 1
btrfs_ioctl() btrfs_ioctl_quota_ctl() btrfs_quota_enable() mutex_lock(fs_info->qgroup_ioctl_lock) btrfs_start_transaction()
btrfs_ioctl() btrfs_ioctl_snap_create_v2 create_snapshot() --> adds snapshot to the list pending_snapshots of the current transaction
btrfs_commit_transaction() create_pending_snapshots() create_pending_snapshot() qgroup_account_snapshot() btrfs_qgroup_inherit() mutex_lock(fs_info->qgroup_ioctl_lock) --> deadlock, mutex already locked by this task at btrfs_quota_enable()
Second scenario:
CPU 0 CPU 1
btrfs_ioctl() btrfs_ioctl_quota_ctl() btrfs_quota_enable() mutex_lock(fs_info->qgroup_ioctl_lock) btrfs_start_transaction()
btrfs_ioctl() btrfs_ioctl_snap_create_v2 create_snapshot() --> adds snapshot to the list pending_snapshots of the current transaction
btrfs_commit_transaction() --> waits for task at CPU 0 to release its transaction handle
btrfs_commit_transaction() --> sees another task started the transaction commit first --> releases its transaction handle --> waits for the transaction commit to be completed by the task at CPU 1
create_pending_snapshot() qgroup_account_snapshot() btrfs_qgroup_inherit() mutex_lock(fs_info->qgroup_ioctl_lock) --> deadlock, task at CPU 0 has the mutex locked but it is waiting for us to finish the transaction commit
So fix this by setting the quota enabled flag in fs_info after committing the transaction at btrfs_quota_enable(). This ends up serializing quota enable and snapshot creation as if the snapshot creation happened just before the quota enable request. The quota rescan task, scheduled after committing the transaction in btrfs_quote_enable(), will do the accounting.
Fixes: 6426c7ad697d ("btrfs: qgroup: Fix qgroup accounting when creating snapshot") Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/qgroup.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-)
--- a/fs/btrfs/qgroup.c +++ b/fs/btrfs/qgroup.c @@ -1013,16 +1013,22 @@ out_add_root: btrfs_abort_transaction(trans, ret); goto out_free_path; } - spin_lock(&fs_info->qgroup_lock); - fs_info->quota_root = quota_root; - set_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags); - spin_unlock(&fs_info->qgroup_lock);
ret = btrfs_commit_transaction(trans); trans = NULL; if (ret) goto out_free_path;
+ /* + * Set quota enabled flag after committing the transaction, to avoid + * deadlocks on fs_info->qgroup_ioctl_lock with concurrent snapshot + * creation. + */ + spin_lock(&fs_info->qgroup_lock); + fs_info->quota_root = quota_root; + set_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags); + spin_unlock(&fs_info->qgroup_lock); + ret = qgroup_rescan_init(fs_info, 0, 1); if (!ret) { qgroup_rescan_zero_tracking(fs_info);