On 8.01.20 г. 7:12 ч., Qu Wenruo wrote:
> [BUG]
> There are several different KASAN reports for balance + snapshot
> workloads.
> Involved call paths include:
>
> should_ignore_root+0x54/0xb0 [btrfs]
> build_backref_tree+0x11af/0x2280 [btrfs]
> relocate_tree_blocks+0x391/0xb80 [btrfs]
> relocate_block_group+0x3e5/0xa00 [btrfs]
> btrfs_relocate_block_group+0x240/0x4d0 [btrfs]
> btrfs_relocate_chunk+0x53/0xf0 [btrfs]
> btrfs_balance+0xc91/0x1840 [btrfs]
> btrfs_ioctl_balance+0x416/0x4e0 [btrfs]
> btrfs_ioctl+0x8af/0x3e60 [btrfs]
> do_vfs_ioctl+0x831/0xb10
> ksys_ioctl+0x67/0x90
> __x64_sys_ioctl+0x43/0x50
> do_syscall_64+0x79/0xe0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> create_reloc_root+0x9f/0x460 [btrfs]
> btrfs_reloc_post_snapshot+0xff/0x6c0 [btrfs]
> create_pending_snapshot+0xa9b/0x15f0 [btrfs]
> create_pending_snapshots+0x111/0x140 [btrfs]
> btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
> btrfs_mksubvol+0x915/0x960 [btrfs]
> btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
> btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
> btrfs_ioctl+0x241b/0x3e60 [btrfs]
> do_vfs_ioctl+0x831/0xb10
> ksys_ioctl+0x67/0x90
> __x64_sys_ioctl+0x43/0x50
> do_syscall_64+0x79/0xe0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> btrfs_reloc_pre_snapshot+0x85/0xc0 [btrfs]
> create_pending_snapshot+0x209/0x15f0 [btrfs]
> create_pending_snapshots+0x111/0x140 [btrfs]
> btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
> btrfs_mksubvol+0x915/0x960 [btrfs]
> btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
> btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
> btrfs_ioctl+0x241b/0x3e60 [btrfs]
> do_vfs_ioctl+0x831/0xb10
> ksys_ioctl+0x67/0x90
> __x64_sys_ioctl+0x43/0x50
> do_syscall_64+0x79/0xe0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> [CAUSE]
> All these call sites are only relying on root->reloc_root, which can
> undergo btrfs_drop_snapshot(), and since we don't have real refcount
what do you mean by "root->reloc_root can undergo btrfs_drop_snapshot" ?
> based protection to reloc roots, we can reach already dropped reloc
> root, triggering KASAN.
what's the relationship between not having a refcount protection and
reaching reloc roots, perhaps you could expand the explanation?
>
> [FIX]
> To avoid such access to unstable root->reloc_root, we should check
> BTRFS_ROOT_DEAD_RELOC_TREE bit first.
>
> This patch introduces a new wrapper, have_reloc_root(), to do the proper
> check for most callers who don't distinguish merged reloc tree and no
> reloc tree.
>
> The only exception is should_ignore_root(), as merged reloc tree can be
> ignored, while no reloc tree shouldn't.
>
> [CRITICAL SECTION ANALYSE]
> Although test_bit()/set_bit()/clear_bit() doesn't imply a barrier, the
> DEAD_RELOC_TREE bit has extra help from transaction as a higher level
> barrier, the lifespan of root::reloc_root and DEAD_RELOC_TREE bit are:
>
> NULL: reloc_root is NULL PTR: reloc_root is not NULL
> 0: DEAD_RELOC_ROOT bit not set DEAD: DEAD_RELOC_ROOT bit set
>
> (NULL, 0) Initial state __
> | /\ Section A
> btrfs_init_reloc_root() \/
> | __
> (PTR, 0) reloc_root initialized /\
> | |
> btrfs_update_reloc_root() | Section B
> | |
> (PTR, DEAD) reloc_root has been merged \/
> | __
> === btrfs_commit_transaction() ====================
> | /\
> clean_dirty_subvols() |
> | | Section C
> (NULL, DEAD) reloc_root cleanup starts \/
> | __
> btrfs_drop_snapshot() /\
> | | Section D
> (NULL, 0) Back to initial state \/
>
> Very have_reloc_root() or test_bit(DEAD_RELOC_ROOT) caller has hold a
^^ Perhaps you meant: Every caller of have_reloc_root or
test_bit(DED_RELOC_ROOT) holds a transaction handle which ensures
modifications in those function are limited to a single transaction?
> transaction handler, so none of such caller can cross transaction
> boundary.
>
> In Section A, every caller just found no DEAD bit, and grab reloc_root.
>
> In the cross section A-B, caller may get no DEAD bit, but since
> reloc_root is still completely valid thus accessing reloc_root is
> completely safe.
>
> No test_bit() caller can cross the boundary of Section B and Section C.
>
> In Section C, every caller found the DEAD bit, so no one will access
> reloc_root.
>
> In the cross section C-D, either caller gets the DEAD bit set, avoiding
> access reloc_root no matter if it's safe or not.
> Or caller get the DEAD bit cleared, then access reloc_root, which is
> already NULL, nothing will be wrong.
>
> Here we need extra memory barrier in cross section C-D, to ensure
> proper memory order between reloc_root and clear_bit().
>
> In Section D, since no DEAD bit and no reloc_root, it's back to initial
> state.
>
> With this lifespan, it should be clear only one memory barrier is
> needed, between setting reloc_root to NULL and clearing DEAD_RELOC_ROOT
> bit.
>
> Reported-by: Zygo Blaxell <ce3g8jdj@xxxxxxxxxxxxxxxxxxxxx>
> Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
> Suggested-by: David Sterba <dsterba@xxxxxxxx>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> ---
> Changelog:
> v2:
> - Add the [CRITICAL SECTION ANALYSE] part
> This gets me into the rabbit hole of memory ordering, but thanks for
> the help from David (initially mentioning the mb hell) and Nikolay
> (for the proper doc), finally I could explain clearly why only
> one mb is needed.
> - Add comment for the only needed memory barrier.
> ---
> fs/btrfs/relocation.c | 32 ++++++++++++++++++++++++++++----
> 1 file changed, 28 insertions(+), 4 deletions(-)
>
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index d897a8e5e430..17a2484f76a5 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -517,6 +517,22 @@ static int update_backref_cache(struct btrfs_trans_handle *trans,
> return 1;
> }
>
> +/*
> + * Check if this subvolume tree has valid reloc(*) tree.
> + *
> + * *: Reloc tree after swap is considered dead, thus not considered as valid.
> + * This is enough for most callers, as they don't distinguish dead reloc
> + * root from no reloc root.
> + * But should_ignore_root() below is a special case.
> + */
> +static bool have_reloc_root(struct btrfs_root *root)
> +{
> + if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
> + return false;
> + if (!root->reloc_root)
> + return false;
> + return true;
> +}
>
> static int should_ignore_root(struct btrfs_root *root)
> {
> @@ -525,6 +541,10 @@ static int should_ignore_root(struct btrfs_root *root)
> if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))
> return 0;
>
> + /* This root has been merged with its reloc tree, we can ignore it */
> + if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
> + return 1;
> +
> reloc_root = root->reloc_root;
> if (!reloc_root)
> return 0;
> @@ -1478,8 +1498,7 @@ int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
> struct btrfs_root_item *root_item;
> int ret;
>
> - if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state) ||
> - !root->reloc_root)
> + if (!have_reloc_root(root))
> goto out;
>
> reloc_root = root->reloc_root;
> @@ -2201,6 +2220,11 @@ static int clean_dirty_subvols(struct reloc_control *rc)
> if (ret2 < 0 && !ret)
> ret = ret2;
> }
> + /*
> + * Need barrier to ensure clear_bit() only happens after
> + * root->reloc_root = NULL.
> + */
> + smp_mb__before_atomic();
> clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
> btrfs_put_fs_root(root);
> } else {
> @@ -4717,7 +4741,7 @@ void btrfs_reloc_pre_snapshot(struct btrfs_pending_snapshot *pending,
> struct btrfs_root *root = pending->root;
> struct reloc_control *rc = root->fs_info->reloc_ctl;
>
> - if (!root->reloc_root || !rc)
> + if (!rc || !have_reloc_root(root))
> return;
>
> if (!rc->merge_reloc_tree)
> @@ -4751,7 +4775,7 @@ int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
> struct reloc_control *rc = root->fs_info->reloc_ctl;
> int ret;
>
> - if (!root->reloc_root || !rc)
> + if (!rc || !have_reloc_root(root))
> return 0;
>
> rc = root->fs_info->reloc_ctl;
>
