On Wed, Oct 24, 2018 at 5:08 AM Josef Bacik <jo...@toxicpanda.com> wrote: > > On Mon, Oct 22, 2018 at 11:05:08PM +0100, Filipe Manana wrote: > > On Mon, Oct 22, 2018 at 8:18 PM Josef Bacik <jo...@toxicpanda.com> wrote: > > > > > > On Mon, Oct 22, 2018 at 08:10:37PM +0100, fdman...@kernel.org wrote: > > > > From: Filipe Manana <fdman...@suse.com> > > > > > > > > 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.nel...@gmail.com> > > > > Link: > > > > https://lore.kernel.org/linux-btrfs/captelenq9x5kowuq+fa7h1r3nsjg8vyith8+ifjurc_duhh...@mail.gmail.com/ > > > > Fixes: 9d66e233c704 ("Btrfs: load free space cache if it exists") > > > > Tested-by: Andrew Nelson <andrew.s.nel...@gmail.com> > > > > Signed-off-by: Filipe Manana <fdman...@suse.com> > > > > > > Great, thanks, > > > > > > Reviewed-by: Josef Bacik <jo...@toxicpanda.com> > > > > So this makes many fstests occasionally fail with aborted transaction > > due to ENOSPC. > > It's late and I haven't verified yet, but I suppose this is because we > > always skip loading the cache regardless of currently being COWing an > > existing leaf or allocating a new one (growing the tree). > > Needs to be fixed. > > > > How about we just use path->search_commit_root? If we're loading the cache we > just want the last committed version, it's not like we read it after we've > written it. Then we can go back to business as usual. Thanks,
Yeah, that works. It was an idea before sending v1 but it felt a bit dirty at the time. Left fstests running overnight using the commit root approach and everything seems fine. Sending a v4. Another alternative, which would solve similar problems for any tree, would be to allow getting a read lock on an already (spin) write locked eb, just like we do for blocking write locks: https://friendpaste.com/6XrGXb5p0RSJGixUFZ8lCt thanks > > Josef
