Re: [PATCH] btrfs: fix lockdep splat in btrfs_alloc_subvolume_writers

[David Sterba <dsterba@suse.cz>]

On Fri, Mar 16, 2018 at 02:36:27PM -0400, jeffm@suse.com wrote:
> From: Jeff Mahoney <jeffm@suse.com>
> 
> While running btrfs/011, I hit the following lockdep splat.
> 
> This is the important bit:
>    pcpu_alloc+0x1ac/0x5e0
>    __percpu_counter_init+0x4e/0xb0
>    btrfs_init_fs_root+0x99/0x1c0 [btrfs]
>    btrfs_get_fs_root.part.54+0x5b/0x150 [btrfs]
>    resolve_indirect_refs+0x130/0x830 [btrfs]
>    find_parent_nodes+0x69e/0xff0 [btrfs]
>    btrfs_find_all_roots_safe+0xa0/0x110 [btrfs]
>    btrfs_find_all_roots+0x50/0x70 [btrfs]
>    btrfs_qgroup_prepare_account_extents+0x53/0x90 [btrfs]
>    btrfs_commit_transaction+0x3ce/0x9b0 [btrfs]
> 
> The percpu_counter_init call in btrfs_alloc_subvolume_writers
> uses GFP_KERNEL, which we can't do during transaction commit.
> 
> This switches it to GFP_NOFS.

> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> ---
>  fs/btrfs/disk-io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 21f34ad0d411..eb6bb3169a9e 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1108,7 +1108,7 @@ static struct btrfs_subvolume_writers *btrfs_alloc_subvolume_writers(void)
>  	if (!writers)
>  		return ERR_PTR(-ENOMEM);
>  
> -	ret = percpu_counter_init(&writers->counter, 0, GFP_KERNEL);
> +	ret = percpu_counter_init(&writers->counter, 0, GFP_NOFS);

A line above the diff context is another allocation that does GFP_NOFS,
so one of the gfp flags were wrong.

Looks like there's another instance where percpu allocates with
GFP_KERNEL: create_space_info that can be called from the path that
allocates chunks, so this also looks like a NOFS candidate.

And in the same function, there's another indirect and hidden GFP_KERNEL
allocation from kobject_init_and_add. So in this case we can't fix all
the gfp problems at the call site and will have to use the scoped
approach eventually.

I haven't found any instance of such lockdep reports in my logs (over a
long period), so it's quite unlikely to end up in the recursive
allocation.

Patch added to next, thanks.