While logging new directory entries, at tree-log.c:log_new_dir_dentries(),
after we call btrfs_search_forward() we get a leaf with a read lock on it,
and without unlocking that leaf we can end up calling btrfs_iget() to get
an inode pointer. The later (btrfs_iget()) can end up doing a read-only
search on the same tree again, if the inode is not in memory already, which
ends up causing a deadlock if some other task in the meanwhile started a
write search on the tree and is attempting to write lock the same leaf
that btrfs_search_forward() locked while holding write locks on upper
levels of the tree blocking the read search from btrfs_iget(). In this
scenario we get a deadlock.

So fix this by releasing the search path before calling btrfs_iget() at
tree-log.c:log_new_dir_dentries().

Example trace of such deadlock:

[ 4077.478852] kworker/u24:10  D ffff88107fc90640     0 14431      2 0x00000000
[ 4077.486752] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
[ 4077.494346]  ffff880ffa56bad0 0000000000000046 0000000000009000 ffff880ffa56bfd8
[ 4077.502629]  ffff880ffa56bfd8 ffff881016ce21c0 ffffffffa06ecb26 ffff88101a5d6138
[ 4077.510915]  ffff880ebb5173b0 ffff880ffa56baf8 ffff880ebb517410 ffff881016ce21c0
[ 4077.519202] Call Trace:
[ 4077.528752]  [<ffffffffa06ed5ed>] ? btrfs_tree_lock+0xdd/0x2f0 [btrfs]
[ 4077.536049]  [<ffffffff81053680>] ? wake_up_atomic_t+0x30/0x30
[ 4077.542574]  [<ffffffffa068cc1f>] ? btrfs_search_slot+0x79f/0xb10 [btrfs]
[ 4077.550171]  [<ffffffffa06a5073>] ? btrfs_lookup_file_extent+0x33/0x40 [btrfs]
[ 4077.558252]  [<ffffffffa06c600b>] ? __btrfs_drop_extents+0x13b/0xdf0 [btrfs]
[ 4077.566140]  [<ffffffffa06fc9e2>] ? add_delayed_data_ref+0xe2/0x150 [btrfs]
[ 4077.573928]  [<ffffffffa06fd629>] ? btrfs_add_delayed_data_ref+0x149/0x1d0 [btrfs]
[ 4077.582399]  [<ffffffffa06cf3c0>] ? __set_extent_bit+0x4c0/0x5c0 [btrfs]
[ 4077.589896]  [<ffffffffa06b4a64>] ? insert_reserved_file_extent.constprop.75+0xa4/0x320 [btrfs]
[ 4077.599632]  [<ffffffffa06b206d>] ? start_transaction+0x8d/0x470 [btrfs]
[ 4077.607134]  [<ffffffffa06bab57>] ? btrfs_finish_ordered_io+0x2e7/0x600 [btrfs]
[ 4077.615329]  [<ffffffff8104cbc2>] ? process_one_work+0x142/0x3d0
[ 4077.622043]  [<ffffffff8104d729>] ? worker_thread+0x109/0x3b0
[ 4077.628459]  [<ffffffff8104d620>] ? manage_workers.isra.26+0x270/0x270
[ 4077.635759]  [<ffffffff81052b0f>] ? kthread+0xaf/0xc0
[ 4077.641404]  [<ffffffff81052a60>] ? kthread_create_on_node+0x110/0x110
[ 4077.648696]  [<ffffffff814a9ac8>] ? ret_from_fork+0x58/0x90
[ 4077.654926]  [<ffffffff81052a60>] ? kthread_create_on_node+0x110/0x110

[ 4078.358087] kworker/u24:15  D ffff88107fcd0640     0 14436      2 0x00000000
[ 4078.365981] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
[ 4078.373574]  ffff880ffa57fad0 0000000000000046 0000000000009000 ffff880ffa57ffd8
[ 4078.381864]  ffff880ffa57ffd8 ffff88103004d0a0 ffffffffa06ecb26 ffff88101a5d6138
[ 4078.390163]  ffff880fbeffc298 ffff880ffa57faf8 ffff880fbeffc2f8 ffff88103004d0a0
[ 4078.398466] Call Trace:
[ 4078.408019]  [<ffffffffa06ed5ed>] ? btrfs_tree_lock+0xdd/0x2f0 [btrfs]
[ 4078.415322]  [<ffffffff81053680>] ? wake_up_atomic_t+0x30/0x30
[ 4078.421844]  [<ffffffffa068cc1f>] ? btrfs_search_slot+0x79f/0xb10 [btrfs]
[ 4078.429438]  [<ffffffffa06a5073>] ? btrfs_lookup_file_extent+0x33/0x40 [btrfs]
[ 4078.437518]  [<ffffffffa06c600b>] ? __btrfs_drop_extents+0x13b/0xdf0 [btrfs]
[ 4078.445404]  [<ffffffffa06fc9e2>] ? add_delayed_data_ref+0xe2/0x150 [btrfs]
[ 4078.453194]  [<ffffffffa06fd629>] ? btrfs_add_delayed_data_ref+0x149/0x1d0 [btrfs]
[ 4078.461663]  [<ffffffffa06cf3c0>] ? __set_extent_bit+0x4c0/0x5c0 [btrfs]
[ 4078.469161]  [<ffffffffa06b4a64>] ? insert_reserved_file_extent.constprop.75+0xa4/0x320 [btrfs]
[ 4078.478893]  [<ffffffffa06b206d>] ? start_transaction+0x8d/0x470 [btrfs]
[ 4078.486388]  [<ffffffffa06bab57>] ? btrfs_finish_ordered_io+0x2e7/0x600 [btrfs]
[ 4078.494561]  [<ffffffff8104cbc2>] ? process_one_work+0x142/0x3d0
[ 4078.501278]  [<ffffffff8104a507>] ? pwq_activate_delayed_work+0x27/0x40
[ 4078.508673]  [<ffffffff8104d729>] ? worker_thread+0x109/0x3b0
[ 4078.515098]  [<ffffffff8104d620>] ? manage_workers.isra.26+0x270/0x270
[ 4078.522396]  [<ffffffff81052b0f>] ? kthread+0xaf/0xc0
[ 4078.528032]  [<ffffffff81052a60>] ? kthread_create_on_node+0x110/0x110
[ 4078.535325]  [<ffffffff814a9ac8>] ? ret_from_fork+0x58/0x90
[ 4078.541552]  [<ffffffff81052a60>] ? kthread_create_on_node+0x110/0x110

[ 4079.355824] user-space-program D ffff88107fd30640     0 32020      1 0x00000000
[ 4079.363716]  ffff880eae8eba10 0000000000000086 0000000000009000 ffff880eae8ebfd8
[ 4079.372003]  ffff880eae8ebfd8 ffff881016c162c0 ffffffffa06ecb26 ffff88101a5d6138
[ 4079.380294]  ffff880fbed4b4c8 ffff880eae8eba38 ffff880fbed4b528 ffff881016c162c0
[ 4079.388586] Call Trace:
[ 4079.398134]  [<ffffffffa06ed595>] ? btrfs_tree_lock+0x85/0x2f0 [btrfs]
[ 4079.405431]  [<ffffffff81053680>] ? wake_up_atomic_t+0x30/0x30
[ 4079.411955]  [<ffffffffa06876fb>] ? btrfs_lock_root_node+0x2b/0x40 [btrfs]
[ 4079.419644]  [<ffffffffa068ce83>] ? btrfs_search_slot+0xa03/0xb10 [btrfs]
[ 4079.427237]  [<ffffffffa06aba52>] ? btrfs_buffer_uptodate+0x52/0x70 [btrfs]
[ 4079.435041]  [<ffffffffa0689b60>] ? generic_bin_search.constprop.38+0x80/0x190 [btrfs]
[ 4079.443897]  [<ffffffffa068ea44>] ? btrfs_insert_empty_items+0x74/0xd0 [btrfs]
[ 4079.451975]  [<ffffffffa072c443>] ? copy_items+0x128/0x850 [btrfs]
[ 4079.458890]  [<ffffffffa072da10>] ? btrfs_log_inode+0x629/0xbf3 [btrfs]
[ 4079.466292]  [<ffffffffa06f34a1>] ? btrfs_log_inode_parent+0xc61/0xf30 [btrfs]
[ 4079.474373]  [<ffffffffa06f45a9>] ? btrfs_log_dentry_safe+0x59/0x80 [btrfs]
[ 4079.482161]  [<ffffffffa06c298d>] ? btrfs_sync_file+0x20d/0x330 [btrfs]
[ 4079.489558]  [<ffffffff8112777c>] ? do_fsync+0x4c/0x80
[ 4079.495300]  [<ffffffff81127a0a>] ? SyS_fdatasync+0xa/0x10
[ 4079.501422]  [<ffffffff814a9b72>] ? system_call_fastpath+0x16/0x1b

[ 4079.508334] user-space-program D ffff88107fc30640     0 32021      1 0x00000004
[ 4079.516226]  ffff880eae8efbf8 0000000000000086 0000000000009000 ffff880eae8effd8
[ 4079.524513]  ffff880eae8effd8 ffff881030279610 ffffffffa06ecb26 ffff88101a5d6138
[ 4079.532802]  ffff880ebb671d88 ffff880eae8efc20 ffff880ebb671de8 ffff881030279610
[ 4079.541092] Call Trace:
[ 4079.550642]  [<ffffffffa06ed595>] ? btrfs_tree_lock+0x85/0x2f0 [btrfs]
[ 4079.557941]  [<ffffffff81053680>] ? wake_up_atomic_t+0x30/0x30
[ 4079.564463]  [<ffffffffa068cc1f>] ? btrfs_search_slot+0x79f/0xb10 [btrfs]
[ 4079.572058]  [<ffffffffa06bb7d8>] ? btrfs_truncate_inode_items+0x168/0xb90 [btrfs]
[ 4079.580526]  [<ffffffffa06b04be>] ? join_transaction.isra.15+0x1e/0x3a0 [btrfs]
[ 4079.588701]  [<ffffffffa06b206d>] ? start_transaction+0x8d/0x470 [btrfs]
[ 4079.596196]  [<ffffffffa0690ac6>] ? block_rsv_add_bytes+0x16/0x50 [btrfs]
[ 4079.603789]  [<ffffffffa06bc2e9>] ? btrfs_truncate+0xe9/0x2e0 [btrfs]
[ 4079.610994]  [<ffffffffa06bd00b>] ? btrfs_setattr+0x30b/0x410 [btrfs]
[ 4079.618197]  [<ffffffff81117c1c>] ? notify_change+0x1dc/0x680
[ 4079.624625]  [<ffffffff8123c8a4>] ? aa_path_perm+0xd4/0x160
[ 4079.630854]  [<ffffffff810f4fcb>] ? do_truncate+0x5b/0x90
[ 4079.636889]  [<ffffffff810f59fa>] ? do_sys_ftruncate.constprop.15+0x10a/0x160
[ 4079.644869]  [<ffffffff8110d87b>] ? SyS_fcntl+0x5b/0x570
[ 4079.650805]  [<ffffffff814a9b72>] ? system_call_fastpath+0x16/0x1b

[ 4080.410607] user-space-program D ffff88107fc70640     0 32028  12639 0x00000004
[ 4080.418489]  ffff880eaeccbbe0 0000000000000086 0000000000009000 ffff880eaeccbfd8
[ 4080.426778]  ffff880eaeccbfd8 ffff880f317ef1e0 ffffffffa06ecb26 ffff88101a5d6138
[ 4080.435067]  ffff880ef7e93928 ffff880f317ef1e0 ffff880eaeccbc08 ffff880f317ef1e0
[ 4080.443353] Call Trace:
[ 4080.452920]  [<ffffffffa06ed15d>] ? btrfs_tree_read_lock+0xdd/0x190 [btrfs]
[ 4080.460703]  [<ffffffff81053680>] ? wake_up_atomic_t+0x30/0x30
[ 4080.467225]  [<ffffffffa06876bb>] ? btrfs_read_lock_root_node+0x2b/0x40 [btrfs]
[ 4080.475400]  [<ffffffffa068cc81>] ? btrfs_search_slot+0x801/0xb10 [btrfs]
[ 4080.482994]  [<ffffffffa06b2df0>] ? btrfs_clean_one_deleted_snapshot+0xe0/0xe0 [btrfs]
[ 4080.491857]  [<ffffffffa06a70a6>] ? btrfs_lookup_inode+0x26/0x90 [btrfs]
[ 4080.499353]  [<ffffffff810ec42f>] ? kmem_cache_alloc+0xaf/0xc0
[ 4080.505879]  [<ffffffffa06bd905>] ? btrfs_iget+0xd5/0x5d0 [btrfs]
[ 4080.512696]  [<ffffffffa06caf04>] ? btrfs_get_token_64+0x104/0x120 [btrfs]
[ 4080.520387]  [<ffffffffa06f341f>] ? btrfs_log_inode_parent+0xbdf/0xf30 [btrfs]
[ 4080.528469]  [<ffffffffa06f45a9>] ? btrfs_log_dentry_safe+0x59/0x80 [btrfs]
[ 4080.536258]  [<ffffffffa06c298d>] ? btrfs_sync_file+0x20d/0x330 [btrfs]
[ 4080.543657]  [<ffffffff8112777c>] ? do_fsync+0x4c/0x80
[ 4080.549399]  [<ffffffff81127a0a>] ? SyS_fdatasync+0xa/0x10
[ 4080.555534]  [<ffffffff814a9b72>] ? system_call_fastpath+0x16/0x1b
Signed-off-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Fixes: 2f2ff0ee (Btrfs: fix metadata inconsistencies after directory fsync)
Cc: stable@vger.kernel.org # 4.1+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
[Modified changelog for clarity and correctness]