From 348a0013d54acec35c22958480af054b97b5e4fe Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Fri, 27 Nov 2015 12:16:16 +0000 Subject: Btrfs: fix unprotected list move from unused_bgs to deleted_bgs list As of my previous change titled "Btrfs: fix scrub preventing unused block groups from being deleted", the following warning at extent-tree.c:btrfs_delete_unused_bgs() can be hit when we mount the a filesysten with "-o discard": 10263 void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info) 10264 { (...) 10405 if (trimming) { 10406 WARN_ON(!list_empty(&block_group->bg_list)); 10407 spin_lock(&trans->transaction->deleted_bgs_lock); 10408 list_move(&block_group->bg_list, 10409 &trans->transaction->deleted_bgs); 10410 spin_unlock(&trans->transaction->deleted_bgs_lock); 10411 btrfs_get_block_group(block_group); 10412 } (...) This happens because scrub can now add back the block group to the list of unused block groups (fs_info->unused_bgs). This is dangerous because we are moving the block group from the unused block groups list to the list of deleted block groups without holding the lock that protects the source list (fs_info->unused_bgs_lock). The following diagram illustrates how this happens: CPU 1 CPU 2 cleaner_kthread() btrfs_delete_unused_bgs() sees bg X in list fs_info->unused_bgs deletes bg X from list fs_info->unused_bgs scrub_enumerate_chunks() searches device tree using its commit root finds device extent for block group X gets block group X from the tree fs_info->block_group_cache_tree (via btrfs_lookup_block_group()) sets bg X to RO (again) scrub_chunk(bg X) sets bg X back to RW mode adds bg X to the list fs_info->unused_bgs again, since it's still unused and currently not in that list sets bg X to RO mode btrfs_remove_chunk(bg X) --> discard is enabled and bg X is in the fs_info->unused_bgs list again so the warning is triggered --> we move it from that list into the transaction's delete_bgs list, but we can have another task currently manipulating the first list (fs_info->unused_bgs) Fix this by using the same lock (fs_info->unused_bgs_lock) to protect both the list of unused block groups and the list of deleted block groups. This makes it safe and there's not much worry for more lock contention, as this lock is seldom used and only the cleaner kthread adds elements to the list of deleted block groups. The warning goes away too, as this was previously an impossible case (and would have been better a BUG_ON/ASSERT) but it's not impossible anymore. Reproduced with fstest btrfs/073 (using MOUNT_OPTIONS="-o discard"). Signed-off-by: Filipe Manana diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 4b89680a..c4661db 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -10480,11 +10480,15 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info) * until transaction commit to do the actual discard. */ if (trimming) { - WARN_ON(!list_empty(&block_group->bg_list)); - spin_lock(&trans->transaction->deleted_bgs_lock); + spin_lock(&fs_info->unused_bgs_lock); + /* + * A concurrent scrub might have added us to the list + * fs_info->unused_bgs, so use a list_move operation + * to add the block group to the deleted_bgs list. + */ list_move(&block_group->bg_list, &trans->transaction->deleted_bgs); - spin_unlock(&trans->transaction->deleted_bgs_lock); + spin_unlock(&fs_info->unused_bgs_lock); btrfs_get_block_group(block_group); } end_trans: diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 3367a3c..be8eae8 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -274,7 +274,6 @@ loop: cur_trans->num_dirty_bgs = 0; spin_lock_init(&cur_trans->dirty_bgs_lock); INIT_LIST_HEAD(&cur_trans->deleted_bgs); - spin_lock_init(&cur_trans->deleted_bgs_lock); spin_lock_init(&cur_trans->dropped_roots_lock); list_add_tail(&cur_trans->list, &fs_info->trans_list); extent_io_tree_init(&cur_trans->dirty_pages, diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h index 0da21ca..64c8221 100644 --- a/fs/btrfs/transaction.h +++ b/fs/btrfs/transaction.h @@ -77,8 +77,8 @@ struct btrfs_transaction { */ struct mutex cache_write_mutex; spinlock_t dirty_bgs_lock; + /* Protected by spin lock fs_info->unused_bgs_lock. */ struct list_head deleted_bgs; - spinlock_t deleted_bgs_lock; spinlock_t dropped_roots_lock; struct btrfs_delayed_ref_root delayed_refs; int aborted; -- cgit v0.10.2