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authorFilipe Manana <fdmanana@suse.com>2018-10-24 10:13:03 +0100
committerDavid Sterba <dsterba@suse.com>2018-11-06 16:42:32 +0100
commit4222ea7100c0e37adace2790c8822758bbeee179 (patch)
treec10f267aaeaf99c03440f87177195f0e42f3e065 /fs/btrfs/free-space-cache.c
parent7e17916b35797396f681a3270245fd29c1e4c250 (diff)
downloadlinux-4222ea7100c0e37adace2790c8822758bbeee179.tar.bz2
Btrfs: fix deadlock on tree root leaf when finding free extent
When we are writing out a free space cache, during the transaction commit phase, we can end up in a deadlock which results in a stack trace like the following: schedule+0x28/0x80 btrfs_tree_read_lock+0x8e/0x120 [btrfs] ? finish_wait+0x80/0x80 btrfs_read_lock_root_node+0x2f/0x40 [btrfs] btrfs_search_slot+0xf6/0x9f0 [btrfs] ? evict_refill_and_join+0xd0/0xd0 [btrfs] ? inode_insert5+0x119/0x190 btrfs_lookup_inode+0x3a/0xc0 [btrfs] ? kmem_cache_alloc+0x166/0x1d0 btrfs_iget+0x113/0x690 [btrfs] __lookup_free_space_inode+0xd8/0x150 [btrfs] lookup_free_space_inode+0x5b/0xb0 [btrfs] load_free_space_cache+0x7c/0x170 [btrfs] ? cache_block_group+0x72/0x3b0 [btrfs] cache_block_group+0x1b3/0x3b0 [btrfs] ? finish_wait+0x80/0x80 find_free_extent+0x799/0x1010 [btrfs] btrfs_reserve_extent+0x9b/0x180 [btrfs] btrfs_alloc_tree_block+0x1b3/0x4f0 [btrfs] __btrfs_cow_block+0x11d/0x500 [btrfs] btrfs_cow_block+0xdc/0x180 [btrfs] btrfs_search_slot+0x3bd/0x9f0 [btrfs] btrfs_lookup_inode+0x3a/0xc0 [btrfs] ? kmem_cache_alloc+0x166/0x1d0 btrfs_update_inode_item+0x46/0x100 [btrfs] cache_save_setup+0xe4/0x3a0 [btrfs] btrfs_start_dirty_block_groups+0x1be/0x480 [btrfs] btrfs_commit_transaction+0xcb/0x8b0 [btrfs] At cache_save_setup() we need to update the inode item of a block group's cache which is located in the tree root (fs_info->tree_root), which means that it may result in COWing a leaf from that tree. If that happens we need to find a free metadata extent and while looking for one, if we find a block group which was not cached yet we attempt to load its cache by calling cache_block_group(). However this function will try to load the inode of the free space cache, which requires finding the matching inode item in the tree root - if that inode item is located in the same leaf as the inode item of the space cache we are updating at cache_save_setup(), we end up in a deadlock, since we try to obtain a read lock on the same extent buffer that we previously write locked. So fix this by using the tree root's commit root when searching for a block group's free space cache inode item when we are attempting to load a free space cache. This is safe since block groups once loaded stay in memory forever, as well as their caches, so after they are first loaded we will never need to read their inode items again. For new block groups, once they are created they get their ->cached field set to BTRFS_CACHE_FINISHED meaning we will not need to read their inode item. Reported-by: Andrew Nelson <andrew.s.nelson@gmail.com> Link: https://lore.kernel.org/linux-btrfs/CAPTELenq9x5KOWuQ+fa7h1r3nsJG8vyiTH8+ifjURc_duHh2Wg@mail.gmail.com/ Fixes: 9d66e233c704 ("Btrfs: load free space cache if it exists") Tested-by: Andrew Nelson <andrew.s.nelson@gmail.com> Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
Diffstat (limited to 'fs/btrfs/free-space-cache.c')
-rw-r--r--fs/btrfs/free-space-cache.c22
1 files changed, 21 insertions, 1 deletions
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 4ba0aedc878b..74aa552f4793 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -75,7 +75,8 @@ static struct inode *__lookup_free_space_inode(struct btrfs_root *root,
* sure NOFS is set to keep us from deadlocking.
*/
nofs_flag = memalloc_nofs_save();
- inode = btrfs_iget(fs_info->sb, &location, root, NULL);
+ inode = btrfs_iget_path(fs_info->sb, &location, root, NULL, path);
+ btrfs_release_path(path);
memalloc_nofs_restore(nofs_flag);
if (IS_ERR(inode))
return inode;
@@ -838,6 +839,25 @@ int load_free_space_cache(struct btrfs_fs_info *fs_info,
path->search_commit_root = 1;
path->skip_locking = 1;
+ /*
+ * We must pass a path with search_commit_root set to btrfs_iget in
+ * order to avoid a deadlock when allocating extents for the tree root.
+ *
+ * When we are COWing an extent buffer from the tree root, when looking
+ * for a free extent, at extent-tree.c:find_free_extent(), we can find
+ * block group without its free space cache loaded. When we find one
+ * we must load its space cache which requires reading its free space
+ * cache's inode item from the root tree. If this inode item is located
+ * in the same leaf that we started COWing before, then we end up in
+ * deadlock on the extent buffer (trying to read lock it when we
+ * previously write locked it).
+ *
+ * It's safe to read the inode item using the commit root because
+ * block groups, once loaded, stay in memory forever (until they are
+ * removed) as well as their space caches once loaded. New block groups
+ * once created get their ->cached field set to BTRFS_CACHE_FINISHED so
+ * we will never try to read their inode item while the fs is mounted.
+ */
inode = lookup_free_space_inode(fs_info, block_group, path);
if (IS_ERR(inode)) {
btrfs_free_path(path);