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Subject[PATCH 5.5 109/150] Btrfs: fix deadlock during fast fsync when logging prealloc extents beyond eof
Date
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;
}

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