This is a note to let you know that I've just added the patch titled
btrfs: take the cleaner_mutex earlier in qgroup disable
to the 6.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
btrfs-take-the-cleaner_mutex-earlier-in-qgroup-disab.patch
and it can be found in the queue-6.8 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 83e6684c0c59079291acc828c6ec90cc9f42305d
Author: Josef Bacik <josef@xxxxxxxxxxxxxx>
Date: Fri Apr 19 14:38:48 2024 -0400
btrfs: take the cleaner_mutex earlier in qgroup disable
[ Upstream commit 0f2b8098d72a93890e69aa24ec549ef4bc34f4db ]
One of my CI runs popped the following lockdep splat
======================================================
WARNING: possible circular locking dependency detected
6.9.0-rc4+ #1 Not tainted
------------------------------------------------------
btrfs/471533 is trying to acquire lock:
ffff92ba46980850 (&fs_info->cleaner_mutex){+.+.}-{3:3}, at: btrfs_quota_disable+0x54/0x4c0
but task is already holding lock:
ffff92ba46980bd0 (&fs_info->subvol_sem){++++}-{3:3}, at: btrfs_ioctl+0x1c8f/0x2600
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&fs_info->subvol_sem){++++}-{3:3}:
down_read+0x42/0x170
btrfs_rename+0x607/0xb00
btrfs_rename2+0x2e/0x70
vfs_rename+0xaf8/0xfc0
do_renameat2+0x586/0x600
__x64_sys_rename+0x43/0x50
do_syscall_64+0x95/0x180
entry_SYSCALL_64_after_hwframe+0x76/0x7e
-> #1 (&sb->s_type->i_mutex_key#16){++++}-{3:3}:
down_write+0x3f/0xc0
btrfs_inode_lock+0x40/0x70
prealloc_file_extent_cluster+0x1b0/0x370
relocate_file_extent_cluster+0xb2/0x720
relocate_data_extent+0x107/0x160
relocate_block_group+0x442/0x550
btrfs_relocate_block_group+0x2cb/0x4b0
btrfs_relocate_chunk+0x50/0x1b0
btrfs_balance+0x92f/0x13d0
btrfs_ioctl+0x1abf/0x2600
__x64_sys_ioctl+0x97/0xd0
do_syscall_64+0x95/0x180
entry_SYSCALL_64_after_hwframe+0x76/0x7e
-> #0 (&fs_info->cleaner_mutex){+.+.}-{3:3}:
__lock_acquire+0x13e7/0x2180
lock_acquire+0xcb/0x2e0
__mutex_lock+0xbe/0xc00
btrfs_quota_disable+0x54/0x4c0
btrfs_ioctl+0x206b/0x2600
__x64_sys_ioctl+0x97/0xd0
do_syscall_64+0x95/0x180
entry_SYSCALL_64_after_hwframe+0x76/0x7e
other info that might help us debug this:
Chain exists of:
&fs_info->cleaner_mutex --> &sb->s_type->i_mutex_key#16 --> &fs_info->subvol_sem
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&fs_info->subvol_sem);
lock(&sb->s_type->i_mutex_key#16);
lock(&fs_info->subvol_sem);
lock(&fs_info->cleaner_mutex);
*** DEADLOCK ***
2 locks held by btrfs/471533:
#0: ffff92ba4319e420 (sb_writers#14){.+.+}-{0:0}, at: btrfs_ioctl+0x3b5/0x2600
#1: ffff92ba46980bd0 (&fs_info->subvol_sem){++++}-{3:3}, at: btrfs_ioctl+0x1c8f/0x2600
stack backtrace:
CPU: 1 PID: 471533 Comm: btrfs Kdump: loaded Not tainted 6.9.0-rc4+ #1
Call Trace:
<TASK>
dump_stack_lvl+0x77/0xb0
check_noncircular+0x148/0x160
? lock_acquire+0xcb/0x2e0
__lock_acquire+0x13e7/0x2180
lock_acquire+0xcb/0x2e0
? btrfs_quota_disable+0x54/0x4c0
? lock_is_held_type+0x9a/0x110
__mutex_lock+0xbe/0xc00
? btrfs_quota_disable+0x54/0x4c0
? srso_return_thunk+0x5/0x5f
? lock_acquire+0xcb/0x2e0
? btrfs_quota_disable+0x54/0x4c0
? btrfs_quota_disable+0x54/0x4c0
btrfs_quota_disable+0x54/0x4c0
btrfs_ioctl+0x206b/0x2600
? srso_return_thunk+0x5/0x5f
? __do_sys_statfs+0x61/0x70
__x64_sys_ioctl+0x97/0xd0
do_syscall_64+0x95/0x180
? srso_return_thunk+0x5/0x5f
? reacquire_held_locks+0xd1/0x1f0
? do_user_addr_fault+0x307/0x8a0
? srso_return_thunk+0x5/0x5f
? lock_acquire+0xcb/0x2e0
? srso_return_thunk+0x5/0x5f
? srso_return_thunk+0x5/0x5f
? find_held_lock+0x2b/0x80
? srso_return_thunk+0x5/0x5f
? lock_release+0xca/0x2a0
? srso_return_thunk+0x5/0x5f
? do_user_addr_fault+0x35c/0x8a0
? srso_return_thunk+0x5/0x5f
? trace_hardirqs_off+0x4b/0xc0
? srso_return_thunk+0x5/0x5f
? lockdep_hardirqs_on_prepare+0xde/0x190
? srso_return_thunk+0x5/0x5f
This happens because when we call rename we already have the inode mutex
held, and then we acquire the subvol_sem if we are a subvolume. This
makes the dependency
inode lock -> subvol sem
When we're running data relocation we will preallocate space for the
data relocation inode, and we always run the relocation under the
->cleaner_mutex. This now creates the dependency of
cleaner_mutex -> inode lock (from the prealloc) -> subvol_sem
Qgroup delete is doing this in the opposite order, it is acquiring the
subvol_sem and then it is acquiring the cleaner_mutex, which results in
this lockdep splat. This deadlock can't happen in reality, because we
won't ever rename the data reloc inode, nor is the data reloc inode a
subvolume.
However this is fairly easy to fix, simply take the cleaner mutex in the
case where we are disabling qgroups before we take the subvol_sem. This
resolves the lockdep splat.
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 6b93fae74403d..8851ba7a1e271 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3722,15 +3722,43 @@ static long btrfs_ioctl_quota_ctl(struct file *file, void __user *arg)
goto drop_write;
}
- down_write(&fs_info->subvol_sem);
-
switch (sa->cmd) {
case BTRFS_QUOTA_CTL_ENABLE:
case BTRFS_QUOTA_CTL_ENABLE_SIMPLE_QUOTA:
+ down_write(&fs_info->subvol_sem);
ret = btrfs_quota_enable(fs_info, sa);
+ up_write(&fs_info->subvol_sem);
break;
case BTRFS_QUOTA_CTL_DISABLE:
+ /*
+ * Lock the cleaner mutex to prevent races with concurrent
+ * relocation, because relocation may be building backrefs for
+ * blocks of the quota root while we are deleting the root. This
+ * is like dropping fs roots of deleted snapshots/subvolumes, we
+ * need the same protection.
+ *
+ * This also prevents races between concurrent tasks trying to
+ * disable quotas, because we will unlock and relock
+ * qgroup_ioctl_lock across BTRFS_FS_QUOTA_ENABLED changes.
+ *
+ * We take this here because we have the dependency of
+ *
+ * inode_lock -> subvol_sem
+ *
+ * because of rename. With relocation we can prealloc extents,
+ * so that makes the dependency chain
+ *
+ * cleaner_mutex -> inode_lock -> subvol_sem
+ *
+ * so we must take the cleaner_mutex here before we take the
+ * subvol_sem. The deadlock can't actually happen, but this
+ * quiets lockdep.
+ */
+ mutex_lock(&fs_info->cleaner_mutex);
+ down_write(&fs_info->subvol_sem);
ret = btrfs_quota_disable(fs_info);
+ up_write(&fs_info->subvol_sem);
+ mutex_unlock(&fs_info->cleaner_mutex);
break;
default:
ret = -EINVAL;
@@ -3738,7 +3766,6 @@ static long btrfs_ioctl_quota_ctl(struct file *file, void __user *arg)
}
kfree(sa);
- up_write(&fs_info->subvol_sem);
drop_write:
mnt_drop_write_file(file);
return ret;
diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 82d4559eb4b1e..cacc12d0ff33f 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1342,16 +1342,10 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
lockdep_assert_held_write(&fs_info->subvol_sem);
/*
- * Lock the cleaner mutex to prevent races with concurrent relocation,
- * because relocation may be building backrefs for blocks of the quota
- * root while we are deleting the root. This is like dropping fs roots
- * of deleted snapshots/subvolumes, we need the same protection.
- *
- * This also prevents races between concurrent tasks trying to disable
- * quotas, because we will unlock and relock qgroup_ioctl_lock across
- * BTRFS_FS_QUOTA_ENABLED changes.
+ * Relocation will mess with backrefs, so make sure we have the
+ * cleaner_mutex held to protect us from relocate.
*/
- mutex_lock(&fs_info->cleaner_mutex);
+ lockdep_assert_held(&fs_info->cleaner_mutex);
mutex_lock(&fs_info->qgroup_ioctl_lock);
if (!fs_info->quota_root)
@@ -1373,9 +1367,13 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
clear_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags);
btrfs_qgroup_wait_for_completion(fs_info, false);
+ /*
+ * We have nothing held here and no trans handle, just return the error
+ * if there is one.
+ */
ret = flush_reservations(fs_info);
if (ret)
- goto out_unlock_cleaner;
+ return ret;
/*
* 1 For the root item
@@ -1439,9 +1437,6 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
btrfs_end_transaction(trans);
else if (trans)
ret = btrfs_commit_transaction(trans);
-out_unlock_cleaner:
- mutex_unlock(&fs_info->cleaner_mutex);
-
return ret;
}
