On Mon, Oct 22, 2018 at 07:48:30PM +0100, fdmanana@xxxxxxxxxx wrote: > From: Filipe Manana <fdmanana@xxxxxxxx> > > 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 skipping the loading of free space caches of any block > groups that are not yet cached (rare cases) if we are COWing an extent > buffer from the root tree and space caching is enabled (-o space_cache > mount option). This is a rare case and its downside is failure to > find a free extent (return -ENOSPC) when all the already cached block > groups have no free extents. > > Reported-by: Andrew Nelson <andrew.s.nelson@xxxxxxxxx> > Link: https://lore.kernel.org/linux-btrfs/CAPTELenq9x5KOWuQ+fa7h1r3nsJG8vyiTH8+ifjURc_duHh2Wg@xxxxxxxxxxxxxx/ > Fixes: 9d66e233c704 ("Btrfs: load free space cache if it exists") > Tested-by: Andrew Nelson <andrew.s.nelson@xxxxxxxxx> > Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx> > --- > > V2: Made the solution more generic, since the problem could happen in any > path COWing an extent buffer from the root tree. > > Applies on top of a previous patch titled: > > "Btrfs: fix deadlock when writing out free space caches" > > fs/btrfs/ctree.c | 4 ++++ > fs/btrfs/ctree.h | 3 +++ > fs/btrfs/disk-io.c | 2 ++ > fs/btrfs/extent-tree.c | 15 ++++++++++++++- > 4 files changed, 23 insertions(+), 1 deletion(-) > > diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c > index 089b46c4d97f..646aafda55a3 100644 > --- a/fs/btrfs/ctree.c > +++ b/fs/btrfs/ctree.c > @@ -1065,10 +1065,14 @@ static noinline int __btrfs_cow_block(struct btrfs_trans_handle *trans, > root == fs_info->chunk_root || > root == fs_info->dev_root) > trans->can_flush_pending_bgs = false; > + else if (root == fs_info->tree_root) > + atomic_inc(&fs_info->tree_root_cows); > > cow = btrfs_alloc_tree_block(trans, root, parent_start, > root->root_key.objectid, &disk_key, level, > search_start, empty_size); > + if (root == fs_info->tree_root) > + atomic_dec(&fs_info->tree_root_cows); Do we need this though? Our root should be the root we're cow'ing the block for, and it should be passed all the way down to find_free_extent properly, so we really should be able to just do if (root == fs_info->tree_root) and not add all this stuff. Not to mention this will race with anybody else doing stuff, so if another thread that isn't actually touching the tree_root it would skip caching a block group when it's completely ok for that thread to do it. Thanks, Josef
