In fstest btrfs/064 a transaction abort in __btrfs_cow_block could lead
to a system lockup. It gets stuck trying to write back inodes, and the
write back thread was trying to lock an extent buffer:

  $ cat /proc/2143497/stack
  [<0>] __btrfs_tree_lock+0x108/0x250
  [<0>] lock_extent_buffer_for_io+0x35e/0x3a0
  [<0>] btree_write_cache_pages+0x15a/0x3b0
  [<0>] do_writepages+0x28/0xb0
  [<0>] __writeback_single_inode+0x54/0x5c0
  [<0>] writeback_sb_inodes+0x1e8/0x510
  [<0>] wb_writeback+0xcc/0x440
  [<0>] wb_workfn+0xd7/0x650
  [<0>] process_one_work+0x236/0x560
  [<0>] worker_thread+0x55/0x3c0
  [<0>] kthread+0x13a/0x150
  [<0>] ret_from_fork+0x1f/0x30

This is because we got an error while COWing a block, specifically here

        if (test_bit(BTRFS_ROOT_SHAREABLE, &root->state)) {
                ret = btrfs_reloc_cow_block(trans, root, buf, cow);
                if (ret) {
                        btrfs_abort_transaction(trans, ret);
                        return ret;
                }
        }

  [16402.241552] BTRFS: Transaction aborted (error -2)
  [16402.242362] WARNING: CPU: 1 PID: 2563188 at fs/btrfs/ctree.c:1074 __btrfs_cow_block+0x376/0x540
  [16402.249469] CPU: 1 PID: 2563188 Comm: fsstress Not tainted 5.9.0-rc6+ #8
  [16402.249936] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
  [16402.250525] RIP: 0010:__btrfs_cow_block+0x376/0x540
  [16402.252417] RSP: 0018:ffff9cca40e578b0 EFLAGS: 00010282
  [16402.252787] RAX: 0000000000000025 RBX: 0000000000000002 RCX: ffff9132bbd19388
  [16402.253278] RDX: 00000000ffffffd8 RSI: 0000000000000027 RDI: ffff9132bbd19380
  [16402.254063] RBP: ffff9132b41a49c0 R08: 0000000000000000 R09: 0000000000000000
  [16402.254887] R10: 0000000000000000 R11: ffff91324758b080 R12: ffff91326ef17ce0
  [16402.255694] R13: ffff91325fc0f000 R14: ffff91326ef176b0 R15: ffff9132815e2000
  [16402.256321] FS:  00007f542c6d7b80(0000) GS:ffff9132bbd00000(0000) knlGS:0000000000000000
  [16402.256973] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [16402.257374] CR2: 00007f127b83f250 CR3: 0000000133480002 CR4: 0000000000370ee0
  [16402.257867] Call Trace:
  [16402.258072]  btrfs_cow_block+0x109/0x230
  [16402.258356]  btrfs_search_slot+0x530/0x9d0
  [16402.258655]  btrfs_lookup_file_extent+0x37/0x40
  [16402.259155]  __btrfs_drop_extents+0x13c/0xd60
  [16402.259628]  ? btrfs_block_rsv_migrate+0x4f/0xb0
  [16402.259949]  btrfs_replace_file_extents+0x190/0x820
  [16402.260873]  btrfs_clone+0x9ae/0xc00
  [16402.261139]  btrfs_extent_same_range+0x66/0x90
  [16402.261771]  btrfs_remap_file_range+0x353/0x3b1
  [16402.262333]  vfs_dedupe_file_range_one.part.0+0xd5/0x140
  [16402.262821]  vfs_dedupe_file_range+0x189/0x220
  [16402.263150]  do_vfs_ioctl+0x552/0x700
  [16402.263662]  __x64_sys_ioctl+0x62/0xb0
  [16402.264023]  do_syscall_64+0x33/0x40
  [16402.264364]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
  [16402.264862] RIP: 0033:0x7f542c7d15cb
  [16402.266901] RSP: 002b:00007ffd35944ea8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [16402.267627] RAX: ffffffffffffffda RBX: 00000000009d1968 RCX: 00007f542c7d15cb
  [16402.268298] RDX: 00000000009d2490 RSI: 00000000c0189436 RDI: 0000000000000003
  [16402.268958] RBP: 00000000009d2520 R08: 0000000000000036 R09: 00000000009d2e64
  [16402.269726] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000002
  [16402.270659] R13: 000000000001f000 R14: 00000000009d1970 R15: 00000000009d2e80
  [16402.271498] irq event stamp: 0
  [16402.271846] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
  [16402.272497] hardirqs last disabled at (0): [<ffffffff910dbf59>] copy_process+0x6b9/0x1ba0
  [16402.273343] softirqs last  enabled at (0): [<ffffffff910dbf59>] copy_process+0x6b9/0x1ba0
  [16402.273905] softirqs last disabled at (0): [<0000000000000000>] 0x0
  [16402.274338] ---[ end trace 737874a5a41a8236 ]---
  [16402.274669] BTRFS: error (device dm-9) in __btrfs_cow_block:1074: errno=-2 No such entry
  [16402.276179] BTRFS info (device dm-9): forced readonly
  [16402.277046] BTRFS: error (device dm-9) in btrfs_replace_file_extents:2723: errno=-2 No such entry
  [16402.278744] BTRFS: error (device dm-9) in __btrfs_cow_block:1074: errno=-2 No such entry
  [16402.279968] BTRFS: error (device dm-9) in __btrfs_cow_block:1074: errno=-2 No such entry
  [16402.280582] BTRFS info (device dm-9): balance: ended with status: -30

The problem here is that as soon as we allocate the new block it is
locked and marked dirty in the btree inode.  This means that we could
attempt to writeback this block and need to lock the extent buffer.
However we're not unlocking it here and thus we deadlock.

Fix this by unlocking the cow block if we have any errors inside of
__btrfs_cow_block, and also free it so we do not leak it.

CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>