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Subject[PATCH AUTOSEL 5.9 19/35] btrfs: add a helper to read the tree_root commit root for backref lookup
Date
From: Josef Bacik <josef@toxicpanda.com>

[ Upstream commit 49d11bead7d596e031fbd34051d8765587cd645b ]

I got the following lockdep splat with tree locks converted to rwsem
patches on btrfs/104:

======================================================
WARNING: possible circular locking dependency detected
5.9.0+ #102 Not tainted
------------------------------------------------------
btrfs-cleaner/903 is trying to acquire lock:
ffff8e7fab6ffe30 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x170

but task is already holding lock:
ffff8e7fab628a88 (&fs_info->commit_root_sem){++++}-{3:3}, at: btrfs_find_all_roots+0x41/0x80

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (&fs_info->commit_root_sem){++++}-{3:3}:
down_read+0x40/0x130
caching_thread+0x53/0x5a0
btrfs_work_helper+0xfa/0x520
process_one_work+0x238/0x540
worker_thread+0x55/0x3c0
kthread+0x13a/0x150
ret_from_fork+0x1f/0x30

-> #2 (&caching_ctl->mutex){+.+.}-{3:3}:
__mutex_lock+0x7e/0x7b0
btrfs_cache_block_group+0x1e0/0x510
find_free_extent+0xb6e/0x12f0
btrfs_reserve_extent+0xb3/0x1b0
btrfs_alloc_tree_block+0xb1/0x330
alloc_tree_block_no_bg_flush+0x4f/0x60
__btrfs_cow_block+0x11d/0x580
btrfs_cow_block+0x10c/0x220
commit_cowonly_roots+0x47/0x2e0
btrfs_commit_transaction+0x595/0xbd0
sync_filesystem+0x74/0x90
generic_shutdown_super+0x22/0x100
kill_anon_super+0x14/0x30
btrfs_kill_super+0x12/0x20
deactivate_locked_super+0x36/0xa0
cleanup_mnt+0x12d/0x190
task_work_run+0x5c/0xa0
exit_to_user_mode_prepare+0x1df/0x200
syscall_exit_to_user_mode+0x54/0x280
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #1 (&space_info->groups_sem){++++}-{3:3}:
down_read+0x40/0x130
find_free_extent+0x2ed/0x12f0
btrfs_reserve_extent+0xb3/0x1b0
btrfs_alloc_tree_block+0xb1/0x330
alloc_tree_block_no_bg_flush+0x4f/0x60
__btrfs_cow_block+0x11d/0x580
btrfs_cow_block+0x10c/0x220
commit_cowonly_roots+0x47/0x2e0
btrfs_commit_transaction+0x595/0xbd0
sync_filesystem+0x74/0x90
generic_shutdown_super+0x22/0x100
kill_anon_super+0x14/0x30
btrfs_kill_super+0x12/0x20
deactivate_locked_super+0x36/0xa0
cleanup_mnt+0x12d/0x190
task_work_run+0x5c/0xa0
exit_to_user_mode_prepare+0x1df/0x200
syscall_exit_to_user_mode+0x54/0x280
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #0 (btrfs-root-00){++++}-{3:3}:
__lock_acquire+0x1167/0x2150
lock_acquire+0xb9/0x3d0
down_read_nested+0x43/0x130
__btrfs_tree_read_lock+0x32/0x170
__btrfs_read_lock_root_node+0x3a/0x50
btrfs_search_slot+0x614/0x9d0
btrfs_find_root+0x35/0x1b0
btrfs_read_tree_root+0x61/0x120
btrfs_get_root_ref+0x14b/0x600
find_parent_nodes+0x3e6/0x1b30
btrfs_find_all_roots_safe+0xb4/0x130
btrfs_find_all_roots+0x60/0x80
btrfs_qgroup_trace_extent_post+0x27/0x40
btrfs_add_delayed_data_ref+0x3fd/0x460
btrfs_free_extent+0x42/0x100
__btrfs_mod_ref+0x1d7/0x2f0
walk_up_proc+0x11c/0x400
walk_up_tree+0xf0/0x180
btrfs_drop_snapshot+0x1c7/0x780
btrfs_clean_one_deleted_snapshot+0xfb/0x110
cleaner_kthread+0xd4/0x140
kthread+0x13a/0x150
ret_from_fork+0x1f/0x30

other info that might help us debug this:

Chain exists of:
btrfs-root-00 --> &caching_ctl->mutex --> &fs_info->commit_root_sem

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&fs_info->commit_root_sem);
lock(&caching_ctl->mutex);
lock(&fs_info->commit_root_sem);
lock(btrfs-root-00);

*** DEADLOCK ***

3 locks held by btrfs-cleaner/903:
#0: ffff8e7fab628838 (&fs_info->cleaner_mutex){+.+.}-{3:3}, at: cleaner_kthread+0x6e/0x140
#1: ffff8e7faadac640 (sb_internal){.+.+}-{0:0}, at: start_transaction+0x40b/0x5c0
#2: ffff8e7fab628a88 (&fs_info->commit_root_sem){++++}-{3:3}, at: btrfs_find_all_roots+0x41/0x80

stack backtrace:
CPU: 0 PID: 903 Comm: btrfs-cleaner Not tainted 5.9.0+ #102
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014
Call Trace:
dump_stack+0x8b/0xb0
check_noncircular+0xcf/0xf0
__lock_acquire+0x1167/0x2150
? __bfs+0x42/0x210
lock_acquire+0xb9/0x3d0
? __btrfs_tree_read_lock+0x32/0x170
down_read_nested+0x43/0x130
? __btrfs_tree_read_lock+0x32/0x170
__btrfs_tree_read_lock+0x32/0x170
__btrfs_read_lock_root_node+0x3a/0x50
btrfs_search_slot+0x614/0x9d0
? find_held_lock+0x2b/0x80
btrfs_find_root+0x35/0x1b0
? do_raw_spin_unlock+0x4b/0xa0
btrfs_read_tree_root+0x61/0x120
btrfs_get_root_ref+0x14b/0x600
find_parent_nodes+0x3e6/0x1b30
btrfs_find_all_roots_safe+0xb4/0x130
btrfs_find_all_roots+0x60/0x80
btrfs_qgroup_trace_extent_post+0x27/0x40
btrfs_add_delayed_data_ref+0x3fd/0x460
btrfs_free_extent+0x42/0x100
__btrfs_mod_ref+0x1d7/0x2f0
walk_up_proc+0x11c/0x400
walk_up_tree+0xf0/0x180
btrfs_drop_snapshot+0x1c7/0x780
? btrfs_clean_one_deleted_snapshot+0x73/0x110
btrfs_clean_one_deleted_snapshot+0xfb/0x110
cleaner_kthread+0xd4/0x140
? btrfs_alloc_root+0x50/0x50
kthread+0x13a/0x150
? kthread_create_worker_on_cpu+0x40/0x40
ret_from_fork+0x1f/0x30
BTRFS info (device sdb): disk space caching is enabled
BTRFS info (device sdb): has skinny extents

This happens because qgroups does a backref lookup when we create a
delayed ref. From here it may have to look up a root from an indirect
ref, which does a normal lookup on the tree_root, which takes the read
lock on the tree_root nodes.

To fix this we need to add a variant for looking up roots that searches
the commit root of the tree_root. Then when we do the backref search
using the commit root we are sure to not take any locks on the tree_root
nodes. This gets rid of the lockdep splat when running btrfs/104.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/backref.c | 13 ++++-
fs/btrfs/disk-io.c | 139 ++++++++++++++++++++++++++++++++-------------
fs/btrfs/disk-io.h | 3 +
3 files changed, 114 insertions(+), 41 deletions(-)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index ea1c28ccb44ff..b948df7a929eb 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -544,7 +544,18 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
int level = ref->level;
struct btrfs_key search_key = ref->key_for_search;

- root = btrfs_get_fs_root(fs_info, ref->root_id, false);
+ /*
+ * If we're search_commit_root we could possibly be holding locks on
+ * other tree nodes. This happens when qgroups does backref walks when
+ * adding new delayed refs. To deal with this we need to look in cache
+ * for the root, and if we don't find it then we need to search the
+ * tree_root's commit root, thus the btrfs_get_fs_root_commit_root usage
+ * here.
+ */
+ if (path->search_commit_root)
+ root = btrfs_get_fs_root_commit_root(fs_info, path, ref->root_id);
+ else
+ root = btrfs_get_fs_root(fs_info, ref->root_id, false);
if (IS_ERR(root)) {
ret = PTR_ERR(root);
goto out_free;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 9f72b092bc228..86344ffc21e93 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1338,32 +1338,26 @@ int btrfs_add_log_tree(struct btrfs_trans_handle *trans,
return 0;
}

-struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
- struct btrfs_key *key)
+static struct btrfs_root *read_tree_root_path(struct btrfs_root *tree_root,
+ struct btrfs_path *path,
+ struct btrfs_key *key)
{
struct btrfs_root *root;
struct btrfs_fs_info *fs_info = tree_root->fs_info;
- struct btrfs_path *path;
u64 generation;
int ret;
int level;

- path = btrfs_alloc_path();
- if (!path)
- return ERR_PTR(-ENOMEM);
-
root = btrfs_alloc_root(fs_info, key->objectid, GFP_NOFS);
- if (!root) {
- ret = -ENOMEM;
- goto alloc_fail;
- }
+ if (!root)
+ return ERR_PTR(-ENOMEM);

ret = btrfs_find_root(tree_root, key, path,
&root->root_item, &root->root_key);
if (ret) {
if (ret > 0)
ret = -ENOENT;
- goto find_fail;
+ goto fail;
}

generation = btrfs_root_generation(&root->root_item);
@@ -1374,21 +1368,31 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
if (IS_ERR(root->node)) {
ret = PTR_ERR(root->node);
root->node = NULL;
- goto find_fail;
+ goto fail;
} else if (!btrfs_buffer_uptodate(root->node, generation, 0)) {
ret = -EIO;
- goto find_fail;
+ goto fail;
}
root->commit_root = btrfs_root_node(root);
-out:
- btrfs_free_path(path);
return root;
-
-find_fail:
+fail:
btrfs_put_root(root);
-alloc_fail:
- root = ERR_PTR(ret);
- goto out;
+ return ERR_PTR(ret);
+}
+
+struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
+ struct btrfs_key *key)
+{
+ struct btrfs_root *root;
+ struct btrfs_path *path;
+
+ path = btrfs_alloc_path();
+ if (!path)
+ return ERR_PTR(-ENOMEM);
+ root = read_tree_root_path(tree_root, path, key);
+ btrfs_free_path(path);
+
+ return root;
}

/*
@@ -1476,6 +1480,31 @@ static struct btrfs_root *btrfs_lookup_fs_root(struct btrfs_fs_info *fs_info,
return root;
}

+static struct btrfs_root *btrfs_get_global_root(struct btrfs_fs_info *fs_info,
+ u64 objectid)
+{
+ if (objectid == BTRFS_ROOT_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->tree_root);
+ if (objectid == BTRFS_EXTENT_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->extent_root);
+ if (objectid == BTRFS_CHUNK_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->chunk_root);
+ if (objectid == BTRFS_DEV_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->dev_root);
+ if (objectid == BTRFS_CSUM_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->csum_root);
+ if (objectid == BTRFS_QUOTA_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->quota_root) ?
+ fs_info->quota_root : ERR_PTR(-ENOENT);
+ if (objectid == BTRFS_UUID_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->uuid_root) ?
+ fs_info->uuid_root : ERR_PTR(-ENOENT);
+ if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID)
+ return btrfs_grab_root(fs_info->free_space_root) ?
+ fs_info->free_space_root : ERR_PTR(-ENOENT);
+ return NULL;
+}
+
int btrfs_insert_fs_root(struct btrfs_fs_info *fs_info,
struct btrfs_root *root)
{
@@ -1573,25 +1602,9 @@ static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info,
struct btrfs_key key;
int ret;

- if (objectid == BTRFS_ROOT_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->tree_root);
- if (objectid == BTRFS_EXTENT_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->extent_root);
- if (objectid == BTRFS_CHUNK_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->chunk_root);
- if (objectid == BTRFS_DEV_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->dev_root);
- if (objectid == BTRFS_CSUM_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->csum_root);
- if (objectid == BTRFS_QUOTA_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->quota_root) ?
- fs_info->quota_root : ERR_PTR(-ENOENT);
- if (objectid == BTRFS_UUID_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->uuid_root) ?
- fs_info->uuid_root : ERR_PTR(-ENOENT);
- if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID)
- return btrfs_grab_root(fs_info->free_space_root) ?
- fs_info->free_space_root : ERR_PTR(-ENOENT);
+ root = btrfs_get_global_root(fs_info, objectid);
+ if (root)
+ return root;
again:
root = btrfs_lookup_fs_root(fs_info, objectid);
if (root) {
@@ -1676,6 +1689,52 @@ struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,
return btrfs_get_root_ref(fs_info, objectid, anon_dev, true);
}

+/*
+ * btrfs_get_fs_root_commit_root - return a root for the given objectid
+ * @fs_info: the fs_info
+ * @objectid: the objectid we need to lookup
+ *
+ * This is exclusively used for backref walking, and exists specifically because
+ * of how qgroups does lookups. Qgroups will do a backref lookup at delayed ref
+ * creation time, which means we may have to read the tree_root in order to look
+ * up a fs root that is not in memory. If the root is not in memory we will
+ * read the tree root commit root and look up the fs root from there. This is a
+ * temporary root, it will not be inserted into the radix tree as it doesn't
+ * have the most uptodate information, it'll simply be discarded once the
+ * backref code is finished using the root.
+ */
+struct btrfs_root *btrfs_get_fs_root_commit_root(struct btrfs_fs_info *fs_info,
+ struct btrfs_path *path,
+ u64 objectid)
+{
+ struct btrfs_root *root;
+ struct btrfs_key key;
+
+ ASSERT(path->search_commit_root && path->skip_locking);
+
+ /*
+ * This can return -ENOENT if we ask for a root that doesn't exist, but
+ * since this is called via the backref walking code we won't be looking
+ * up a root that doesn't exist, unless there's corruption. So if root
+ * != NULL just return it.
+ */
+ root = btrfs_get_global_root(fs_info, objectid);
+ if (root)
+ return root;
+
+ root = btrfs_lookup_fs_root(fs_info, objectid);
+ if (root)
+ return root;
+
+ key.objectid = objectid;
+ key.type = BTRFS_ROOT_ITEM_KEY;
+ key.offset = (u64)-1;
+ root = read_tree_root_path(fs_info->tree_root, path, &key);
+ btrfs_release_path(path);
+
+ return root;
+}
+
/*
* called by the kthread helper functions to finally call the bio end_io
* functions. This is where read checksum verification actually happens
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index 00dc39d47ed34..2e6da9ae8462f 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -69,6 +69,9 @@ struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,
u64 objectid, bool check_ref);
struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,
u64 objectid, dev_t anon_dev);
+struct btrfs_root *btrfs_get_fs_root_commit_root(struct btrfs_fs_info *fs_info,
+ struct btrfs_path *path,
+ u64 objectid);

void btrfs_free_fs_info(struct btrfs_fs_info *fs_info);
int btrfs_cleanup_fs_roots(struct btrfs_fs_info *fs_info);
--
2.27.0
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