The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From fcc99734d1d4ced30167eb02e17f656735cb9928 Mon Sep 17 00:00:00 2001
From: Qu Wenruo wqu@suse.com Date: Mon, 27 Apr 2020 14:50:14 +0800 Subject: [PATCH] btrfs: transaction: Avoid deadlock due to bad initialization timing of fs_info::journal_info
[BUG] One run of btrfs/063 triggered the following lockdep warning: ============================================ WARNING: possible recursive locking detected 5.6.0-rc7-custom+ #48 Not tainted -------------------------------------------- kworker/u24:0/7 is trying to acquire lock: ffff88817d3a46e0 (sb_internal#2){.+.+}, at: start_transaction+0x66c/0x890 [btrfs]
but task is already holding lock: ffff88817d3a46e0 (sb_internal#2){.+.+}, at: start_transaction+0x66c/0x890 [btrfs]
other info that might help us debug this: Possible unsafe locking scenario:
CPU0 ---- lock(sb_internal#2); lock(sb_internal#2);
*** DEADLOCK ***
May be due to missing lock nesting notation
4 locks held by kworker/u24:0/7: #0: ffff88817b495948 ((wq_completion)btrfs-endio-write){+.+.}, at: process_one_work+0x557/0xb80 #1: ffff888189ea7db8 ((work_completion)(&work->normal_work)){+.+.}, at: process_one_work+0x557/0xb80 #2: ffff88817d3a46e0 (sb_internal#2){.+.+}, at: start_transaction+0x66c/0x890 [btrfs] #3: ffff888174ca4da8 (&fs_info->reloc_mutex){+.+.}, at: btrfs_record_root_in_trans+0x83/0xd0 [btrfs]
stack backtrace: CPU: 0 PID: 7 Comm: kworker/u24:0 Not tainted 5.6.0-rc7-custom+ #48 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 Workqueue: btrfs-endio-write btrfs_work_helper [btrfs] Call Trace: dump_stack+0xc2/0x11a __lock_acquire.cold+0xce/0x214 lock_acquire+0xe6/0x210 __sb_start_write+0x14e/0x290 start_transaction+0x66c/0x890 [btrfs] btrfs_join_transaction+0x1d/0x20 [btrfs] find_free_extent+0x1504/0x1a50 [btrfs] btrfs_reserve_extent+0xd5/0x1f0 [btrfs] btrfs_alloc_tree_block+0x1ac/0x570 [btrfs] btrfs_copy_root+0x213/0x580 [btrfs] create_reloc_root+0x3bd/0x470 [btrfs] btrfs_init_reloc_root+0x2d2/0x310 [btrfs] record_root_in_trans+0x191/0x1d0 [btrfs] btrfs_record_root_in_trans+0x90/0xd0 [btrfs] start_transaction+0x16e/0x890 [btrfs] btrfs_join_transaction+0x1d/0x20 [btrfs] btrfs_finish_ordered_io+0x55d/0xcd0 [btrfs] finish_ordered_fn+0x15/0x20 [btrfs] btrfs_work_helper+0x116/0x9a0 [btrfs] process_one_work+0x632/0xb80 worker_thread+0x80/0x690 kthread+0x1a3/0x1f0 ret_from_fork+0x27/0x50
It's pretty hard to reproduce, only one hit so far.
[CAUSE] This is because we're calling btrfs_join_transaction() without re-using the current running one:
btrfs_finish_ordered_io() |- btrfs_join_transaction() <<< Call #1 |- btrfs_record_root_in_trans() |- btrfs_reserve_extent() |- btrfs_join_transaction() <<< Call #2
Normally such btrfs_join_transaction() call should re-use the existing one, without trying to re-start a transaction.
But the problem is, in btrfs_join_transaction() call #1, we call btrfs_record_root_in_trans() before initializing current::journal_info.
And in btrfs_join_transaction() call #2, we're relying on current::journal_info to avoid such deadlock.
[FIX] Call btrfs_record_root_in_trans() after we have initialized current::journal_info.
CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Qu Wenruo wqu@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 8cede6eb9843..2d5498136e5e 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -662,10 +662,19 @@ start_transaction(struct btrfs_root *root, unsigned int num_items, }
got_it: - btrfs_record_root_in_trans(h, root); - if (!current->journal_info) current->journal_info = h; + + /* + * btrfs_record_root_in_trans() needs to alloc new extents, and may + * call btrfs_join_transaction() while we're also starting a + * transaction. + * + * Thus it need to be called after current->journal_info initialized, + * or we can deadlock. + */ + btrfs_record_root_in_trans(h, root); + return h;
join_fail: