Re: [PATCH v4] Btrfs: fix deadlock on tree root leaf when finding free extent

[Filipe Manana <fdmanana@kernel.org>]

On Wed, Oct 24, 2018 at 1:40 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> On Wed, Oct 24, 2018 at 12:53:59PM +0100, Filipe Manana wrote:
> > On Wed, Oct 24, 2018 at 12:37 PM Josef Bacik <josef@toxicpanda.com> wrote:
> > >
> > > On Wed, Oct 24, 2018 at 10:13:03AM +0100, fdmanana@kernel.org wrote:
> > > > From: Filipe Manana <fdmanana@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 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>
> > > > Signed-off-by: Filipe Manana <fdmanana@suse.com>
> > > > ---
> > > >
> > >
> > > Now my goal is to see how many times I can get you to redo this thing.
> > >
> > > Why not instead just do
> > >
> > > if (btrfs_is_free_space_inode(inode))
> > >   path->search_commit_root = 1;
> > >
> > > in read_locked_inode?  That would be cleaner.  If we don't want to do that for
> > > the inode cache (I'm not sure if it's ok or not) we could just do
> > >
> > > if (root == fs_info->tree_root)
> >
> > We can't (not just that at least).
> > Tried something like that, but we get into a BUG_ON when writing out
> > the space cache for new block groups (created in the current
> > transaction).
> > Because at cache_save_setup() we have this:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/extent-tree.c?h=v4.19#n3342
> >
> > Lookup for the inode in normal root, doesn't exist, create it then
> > repeat - if still not found, BUG_ON.
> > Could also make create_free_space_inode() return an inode pointer and
> > make it call btrfs_iget().
> >
>
> Ah ok makes sense.  Well in that case lets just make btrfs_read_locked_inode()
> take a path, and allocate it in btrfs_iget, that'll remove the ugly
>
> if (path != in_path)

You mean the following on top of v4:

https://friendpaste.com/6XrGXb5p0RSJGixUFYouHg

Not much different, just saves one such if statement. I'm ok with that.

>
> stuff.  Thanks,
>
> Josef