On 1/8/20 12:12 AM, 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 based protection to reloc roots, we can reach already dropped reloc root, triggering KASAN. [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 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;
You still need a smp_mb__after_atomic() here, test_bit is unordered. Thanks, Josef
