From: Robbie Ko Date: Thu, 11 Dec 2025 05:30:33 +0000 (+0800) Subject: btrfs: fix deadlock in wait_current_trans() due to ignored transaction type X-Git-Tag: v6.19-rc5~33^2~5 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5037b342825df7094a4906d1e2a9674baab50cb2;p=thirdparty%2Fkernel%2Flinux.git btrfs: fix deadlock in wait_current_trans() due to ignored transaction type When wait_current_trans() is called during start_transaction(), it currently waits for a blocked transaction without considering whether the given transaction type actually needs to wait for that particular transaction state. The btrfs_blocked_trans_types[] array already defines which transaction types should wait for which transaction states, but this check was missing in wait_current_trans(). This can lead to a deadlock scenario involving two transactions and pending ordered extents: 1. Transaction A is in TRANS_STATE_COMMIT_DOING state 2. A worker processing an ordered extent calls start_transaction() with TRANS_JOIN 3. join_transaction() returns -EBUSY because Transaction A is in TRANS_STATE_COMMIT_DOING 4. Transaction A moves to TRANS_STATE_UNBLOCKED and completes 5. A new Transaction B is created (TRANS_STATE_RUNNING) 6. The ordered extent from step 2 is added to Transaction B's pending ordered extents 7. Transaction B immediately starts commit by another task and enters TRANS_STATE_COMMIT_START 8. The worker finally reaches wait_current_trans(), sees Transaction B in TRANS_STATE_COMMIT_START (a blocked state), and waits unconditionally 9. However, TRANS_JOIN should NOT wait for TRANS_STATE_COMMIT_START according to btrfs_blocked_trans_types[] 10. Transaction B is waiting for pending ordered extents to complete 11. Deadlock: Transaction B waits for ordered extent, ordered extent waits for Transaction B This can be illustrated by the following call stacks: CPU0 CPU1 btrfs_finish_ordered_io() start_transaction(TRANS_JOIN) join_transaction() # -EBUSY (Transaction A is # TRANS_STATE_COMMIT_DOING) # Transaction A completes # Transaction B created # ordered extent added to # Transaction B's pending list btrfs_commit_transaction() # Transaction B enters # TRANS_STATE_COMMIT_START # waiting for pending ordered # extents wait_current_trans() # waits for Transaction B # (should not wait!) Task bstore_kv_sync in btrfs_commit_transaction waiting for ordered extents: __schedule+0x2e7/0x8a0 schedule+0x64/0xe0 btrfs_commit_transaction+0xbf7/0xda0 [btrfs] btrfs_sync_file+0x342/0x4d0 [btrfs] __x64_sys_fdatasync+0x4b/0x80 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 Task kworker in wait_current_trans waiting for transaction commit: Workqueue: btrfs-syno_nocow btrfs_work_helper [btrfs] __schedule+0x2e7/0x8a0 schedule+0x64/0xe0 wait_current_trans+0xb0/0x110 [btrfs] start_transaction+0x346/0x5b0 [btrfs] btrfs_finish_ordered_io.isra.0+0x49b/0x9c0 [btrfs] btrfs_work_helper+0xe8/0x350 [btrfs] process_one_work+0x1d3/0x3c0 worker_thread+0x4d/0x3e0 kthread+0x12d/0x150 ret_from_fork+0x1f/0x30 Fix this by passing the transaction type to wait_current_trans() and checking btrfs_blocked_trans_types[cur_trans->state] against the given type before deciding to wait. This ensures that transaction types which are allowed to join during certain blocked states will not unnecessarily wait and cause deadlocks. Reviewed-by: Filipe Manana Signed-off-by: Robbie Ko Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 05ee4391c83a..bd03f465e2d3 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -520,13 +520,14 @@ static inline int is_transaction_blocked(struct btrfs_transaction *trans) * when this is done, it is safe to start a new transaction, but the current * transaction might not be fully on disk. */ -static void wait_current_trans(struct btrfs_fs_info *fs_info) +static void wait_current_trans(struct btrfs_fs_info *fs_info, unsigned int type) { struct btrfs_transaction *cur_trans; spin_lock(&fs_info->trans_lock); cur_trans = fs_info->running_transaction; - if (cur_trans && is_transaction_blocked(cur_trans)) { + if (cur_trans && is_transaction_blocked(cur_trans) && + (btrfs_blocked_trans_types[cur_trans->state] & type)) { refcount_inc(&cur_trans->use_count); spin_unlock(&fs_info->trans_lock); @@ -701,12 +702,12 @@ again: sb_start_intwrite(fs_info->sb); if (may_wait_transaction(fs_info, type)) - wait_current_trans(fs_info); + wait_current_trans(fs_info, type); do { ret = join_transaction(fs_info, type); if (ret == -EBUSY) { - wait_current_trans(fs_info); + wait_current_trans(fs_info, type); if (unlikely(type == TRANS_ATTACH || type == TRANS_JOIN_NOSTART)) ret = -ENOENT; @@ -1003,7 +1004,7 @@ out: void btrfs_throttle(struct btrfs_fs_info *fs_info) { - wait_current_trans(fs_info); + wait_current_trans(fs_info, TRANS_START); } bool btrfs_should_end_transaction(struct btrfs_trans_handle *trans)