6.12-stable review patch. If anyone has any objections, please let me know.
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From: Omar Sandoval osandov@fb.com
commit f6a6c280059c4ddc23e12e3de1b01098e240036f upstream.
There is a race condition between inode eviction and inode caching that can cause a live struct btrfs_inode to be missing from the root->inodes xarray. Specifically, there is a window during evict() between the inode being unhashed and deleted from the xarray. If btrfs_iget() is called for the same inode in that window, it will be recreated and inserted into the xarray, but then eviction will delete the new entry, leaving nothing in the xarray:
Thread 1 Thread 2 --------------------------------------------------------------- evict() remove_inode_hash() btrfs_iget_path() btrfs_iget_locked() btrfs_read_locked_inode() btrfs_add_inode_to_root() destroy_inode() btrfs_destroy_inode() btrfs_del_inode_from_root() __xa_erase
In turn, this can cause issues for subvolume deletion. Specifically, if an inode is in this lost state, and all other inodes are evicted, then btrfs_del_inode_from_root() will call btrfs_add_dead_root() prematurely. If the lost inode has a delayed_node attached to it, then when btrfs_clean_one_deleted_snapshot() calls btrfs_kill_all_delayed_nodes(), it will loop forever because the delayed_nodes xarray will never become empty (unless memory pressure forces the inode out). We saw this manifest as soft lockups in production.
Fix it by only deleting the xarray entry if it matches the given inode (using __xa_cmpxchg()).
Fixes: 310b2f5d5a94 ("btrfs: use an xarray to track open inodes in a root") Cc: stable@vger.kernel.org # 6.11+ Reviewed-by: Josef Bacik josef@toxicpanda.com Reviewed-by: Filipe Manana fdmanana@suse.com Co-authored-by: Leo Martins loemra.dev@gmail.com Signed-off-by: Leo Martins loemra.dev@gmail.com Signed-off-by: Omar Sandoval osandov@fb.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/inode.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)
--- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -5634,7 +5634,17 @@ static void btrfs_del_inode_from_root(st bool empty = false;
xa_lock(&root->inodes); - entry = __xa_erase(&root->inodes, btrfs_ino(inode)); + /* + * This btrfs_inode is being freed and has already been unhashed at this + * point. It's possible that another btrfs_inode has already been + * allocated for the same inode and inserted itself into the root, so + * don't delete it in that case. + * + * Note that this shouldn't need to allocate memory, so the gfp flags + * don't really matter. + */ + entry = __xa_cmpxchg(&root->inodes, btrfs_ino(inode), inode, NULL, + GFP_ATOMIC); if (entry == inode) empty = xa_empty(&root->inodes); xa_unlock(&root->inodes);