From: Filipe Manana fdmanana@suse.com
commit a5ae50dea9111db63d30d700766dd5509602f7ad upstream.
While logging the prealloc extents of an inode during a fast fsync we call btrfs_truncate_inode_items(), through btrfs_log_prealloc_extents(), while holding a read lock on a leaf of the inode's root (not the log root, the fs/subvol root), and then that function locks the file range in the inode's iotree. This can lead to a deadlock when:
* the fsync is ranged
* the file has prealloc extents beyond eof
* writeback for a range different from the fsync range starts during the fsync
* the size of the file is not sector size aligned
Because when finishing an ordered extent we lock first a file range and then try to COW the fs/subvol tree to insert an extent item.
The following diagram shows how the deadlock can happen.
CPU 1 CPU 2
btrfs_sync_file() --> for range [0, 1MiB)
--> inode has a size of 1MiB and has 1 prealloc extent beyond the i_size, starting at offset 4MiB
flushes all delalloc for the range [0MiB, 1MiB) and waits for the respective ordered extents to complete
--> before task at CPU 1 locks the inode, a write into file range [1MiB, 2MiB + 1KiB) is made
--> i_size is updated to 2MiB + 1KiB
--> writeback is started for that range, [1MiB, 2MiB + 4KiB) --> end offset rounded up to be sector size aligned
btrfs_log_dentry_safe() btrfs_log_inode_parent() btrfs_log_inode()
btrfs_log_changed_extents() btrfs_log_prealloc_extents() --> does a search on the inode's root --> holds a read lock on leaf X
btrfs_finish_ordered_io() --> locks range [1MiB, 2MiB + 4KiB) --> end offset rounded up to be sector size aligned
--> tries to cow leaf X, through insert_reserved_file_extent() --> already locked by the task at CPU 1
btrfs_truncate_inode_items()
--> gets an i_size of 2MiB + 1KiB, which is not sector size aligned
--> tries to lock file range [2MiB, (u64)-1) --> the start range is rounded down from 2MiB + 1K to 2MiB to be sector size aligned
--> but the subrange [2MiB, 2MiB + 4KiB) is already locked by task at CPU 2 which is waiting to get a write lock on leaf X for which we are holding a read lock
*** deadlock ***
This results in a stack trace like the following, triggered by test case generic/561 from fstests:
[ 2779.973608] INFO: task kworker/u8:6:247 blocked for more than 120 seconds. [ 2779.979536] Not tainted 5.6.0-rc2-btrfs-next-53 #1 [ 2779.984503] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 2779.990136] kworker/u8:6 D 0 247 2 0x80004000 [ 2779.990457] Workqueue: btrfs-endio-write btrfs_work_helper [btrfs] [ 2779.990466] Call Trace: [ 2779.990491] ? __schedule+0x384/0xa30 [ 2779.990521] schedule+0x33/0xe0 [ 2779.990616] btrfs_tree_read_lock+0x19e/0x2e0 [btrfs] [ 2779.990632] ? remove_wait_queue+0x60/0x60 [ 2779.990730] btrfs_read_lock_root_node+0x2f/0x40 [btrfs] [ 2779.990782] btrfs_search_slot+0x510/0x1000 [btrfs] [ 2779.990869] btrfs_lookup_file_extent+0x4a/0x70 [btrfs] [ 2779.990944] __btrfs_drop_extents+0x161/0x1060 [btrfs] [ 2779.990987] ? mark_held_locks+0x6d/0xc0 [ 2779.990994] ? __slab_alloc.isra.49+0x99/0x100 [ 2779.991060] ? insert_reserved_file_extent.constprop.19+0x64/0x300 [btrfs] [ 2779.991145] insert_reserved_file_extent.constprop.19+0x97/0x300 [btrfs] [ 2779.991222] ? start_transaction+0xdd/0x5c0 [btrfs] [ 2779.991291] btrfs_finish_ordered_io+0x4f4/0x840 [btrfs] [ 2779.991405] btrfs_work_helper+0xaa/0x720 [btrfs] [ 2779.991432] process_one_work+0x26d/0x6a0 [ 2779.991460] worker_thread+0x4f/0x3e0 [ 2779.991481] ? process_one_work+0x6a0/0x6a0 [ 2779.991489] kthread+0x103/0x140 [ 2779.991499] ? kthread_create_worker_on_cpu+0x70/0x70 [ 2779.991515] ret_from_fork+0x3a/0x50 (...) [ 2780.026211] INFO: task fsstress:17375 blocked for more than 120 seconds. [ 2780.027480] Not tainted 5.6.0-rc2-btrfs-next-53 #1 [ 2780.028482] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 2780.030035] fsstress D 0 17375 17373 0x00004000 [ 2780.030038] Call Trace: [ 2780.030044] ? __schedule+0x384/0xa30 [ 2780.030052] schedule+0x33/0xe0 [ 2780.030075] lock_extent_bits+0x20c/0x320 [btrfs] [ 2780.030094] ? btrfs_truncate_inode_items+0xf4/0x1150 [btrfs] [ 2780.030098] ? rcu_read_lock_sched_held+0x59/0xa0 [ 2780.030102] ? remove_wait_queue+0x60/0x60 [ 2780.030122] btrfs_truncate_inode_items+0x133/0x1150 [btrfs] [ 2780.030151] ? btrfs_set_path_blocking+0xb2/0x160 [btrfs] [ 2780.030165] ? btrfs_search_slot+0x379/0x1000 [btrfs] [ 2780.030195] btrfs_log_changed_extents.isra.8+0x841/0x93e [btrfs] [ 2780.030202] ? do_raw_spin_unlock+0x49/0xc0 [ 2780.030215] ? btrfs_get_num_csums+0x10/0x10 [btrfs] [ 2780.030239] btrfs_log_inode+0xf83/0x1124 [btrfs] [ 2780.030251] ? __mutex_unlock_slowpath+0x45/0x2a0 [ 2780.030275] btrfs_log_inode_parent+0x2a0/0xe40 [btrfs] [ 2780.030282] ? dget_parent+0xa1/0x370 [ 2780.030309] btrfs_log_dentry_safe+0x4a/0x70 [btrfs] [ 2780.030329] btrfs_sync_file+0x3f3/0x490 [btrfs] [ 2780.030339] do_fsync+0x38/0x60 [ 2780.030343] __x64_sys_fdatasync+0x13/0x20 [ 2780.030345] do_syscall_64+0x5c/0x280 [ 2780.030348] entry_SYSCALL_64_after_hwframe+0x49/0xbe [ 2780.030356] RIP: 0033:0x7f2d80f6d5f0 [ 2780.030361] Code: Bad RIP value. [ 2780.030362] RSP: 002b:00007ffdba3c8548 EFLAGS: 00000246 ORIG_RAX: 000000000000004b [ 2780.030364] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f2d80f6d5f0 [ 2780.030365] RDX: 00007ffdba3c84b0 RSI: 00007ffdba3c84b0 RDI: 0000000000000003 [ 2780.030367] RBP: 000000000000004a R08: 0000000000000001 R09: 00007ffdba3c855c [ 2780.030368] R10: 0000000000000078 R11: 0000000000000246 R12: 00000000000001f4 [ 2780.030369] R13: 0000000051eb851f R14: 00007ffdba3c85f0 R15: 0000557a49220d90
So fix this by making btrfs_truncate_inode_items() not lock the range in the inode's iotree when the target root is a log root, since it's not needed to lock the range for log roots as the protection from the inode's lock and log_mutex are all that's needed.
Fixes: 28553fa992cb28 ("Btrfs: fix race between shrinking truncate and fiemap") CC: stable@vger.kernel.org # 4.4+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/inode.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
--- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -4775,8 +4775,9 @@ int btrfs_truncate_inode_items(struct bt return -ENOMEM; path->reada = READA_BACK;
- lock_extent_bits(&BTRFS_I(inode)->io_tree, lock_start, (u64)-1, - &cached_state); + if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID) + lock_extent_bits(&BTRFS_I(inode)->io_tree, lock_start, (u64)-1, + &cached_state);
/* * We want to drop from the next block forward in case this new size is @@ -5040,11 +5041,10 @@ out: if (!ret && last_size > new_size) last_size = new_size; btrfs_ordered_update_i_size(inode, last_size, NULL); + unlock_extent_cached(&BTRFS_I(inode)->io_tree, lock_start, + (u64)-1, &cached_state); }
- unlock_extent_cached(&BTRFS_I(inode)->io_tree, lock_start, (u64)-1, - &cached_state); - btrfs_free_path(path); return ret; }