Often some test cases like btrfs/161 trigger lockdep splats that complain
about possible unsafe lock scenario due to the fact that during mount,
when reading the chunk tree we end up calling blkdev_get_by_path() while
holding a read lock on a leaf of the chunk tree. That produces a lockdep
splat like the following:

[ 3653.683975] ======================================================
[ 3653.685148] WARNING: possible circular locking dependency detected
[ 3653.686301] 5.15.0-rc7-btrfs-next-103 #1 Not tainted
[ 3653.687239] ------------------------------------------------------
[ 3653.688400] mount/447465 is trying to acquire lock:
[ 3653.689320] ffff8c6b0c76e528 (&disk->open_mutex){+.+.}-{3:3}, at: blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.691054]
               but task is already holding lock:
[ 3653.692155] ffff8c6b0a9f39e0 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x110 [btrfs]
[ 3653.693978]
               which lock already depends on the new lock.

[ 3653.695510]
               the existing dependency chain (in reverse order) is:
[ 3653.696915]
               -> #3 (btrfs-chunk-00){++++}-{3:3}:
[ 3653.698053]        down_read_nested+0x4b/0x140
[ 3653.698893]        __btrfs_tree_read_lock+0x24/0x110 [btrfs]
[ 3653.699988]        btrfs_read_lock_root_node+0x31/0x40 [btrfs]
[ 3653.701205]        btrfs_search_slot+0x537/0xc00 [btrfs]
[ 3653.702234]        btrfs_insert_empty_items+0x32/0x70 [btrfs]
[ 3653.703332]        btrfs_init_new_device+0x563/0x15b0 [btrfs]
[ 3653.704439]        btrfs_ioctl+0x2110/0x3530 [btrfs]
[ 3653.705405]        __x64_sys_ioctl+0x83/0xb0
[ 3653.706215]        do_syscall_64+0x3b/0xc0
[ 3653.706990]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 3653.708040]
               -> #2 (sb_internal#2){.+.+}-{0:0}:
[ 3653.708994]        lock_release+0x13d/0x4a0
[ 3653.709533]        up_write+0x18/0x160
[ 3653.710017]        btrfs_sync_file+0x3f3/0x5b0 [btrfs]
[ 3653.710699]        __loop_update_dio+0xbd/0x170 [loop]
[ 3653.711360]        lo_ioctl+0x3b1/0x8a0 [loop]
[ 3653.711929]        block_ioctl+0x48/0x50
[ 3653.712442]        __x64_sys_ioctl+0x83/0xb0
[ 3653.712991]        do_syscall_64+0x3b/0xc0
[ 3653.713519]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 3653.714233]
               -> #1 (&lo->lo_mutex){+.+.}-{3:3}:
[ 3653.715026]        __mutex_lock+0x92/0x900
[ 3653.715648]        lo_open+0x28/0x60 [loop]
[ 3653.716275]        blkdev_get_whole+0x28/0x90
[ 3653.716867]        blkdev_get_by_dev.part.0+0x142/0x320
[ 3653.717537]        blkdev_open+0x5e/0xa0
[ 3653.718043]        do_dentry_open+0x163/0x390
[ 3653.718604]        path_openat+0x3f0/0xa80
[ 3653.719128]        do_filp_open+0xa9/0x150
[ 3653.719652]        do_sys_openat2+0x97/0x160
[ 3653.720197]        __x64_sys_openat+0x54/0x90
[ 3653.720766]        do_syscall_64+0x3b/0xc0
[ 3653.721285]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 3653.721986]
               -> #0 (&disk->open_mutex){+.+.}-{3:3}:
[ 3653.722775]        __lock_acquire+0x130e/0x2210
[ 3653.723348]        lock_acquire+0xd7/0x310
[ 3653.723867]        __mutex_lock+0x92/0x900
[ 3653.724394]        blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.725041]        blkdev_get_by_path+0xb8/0xd0
[ 3653.725614]        btrfs_get_bdev_and_sb+0x1b/0xb0 [btrfs]
[ 3653.726332]        open_fs_devices+0xd7/0x2c0 [btrfs]
[ 3653.726999]        btrfs_read_chunk_tree+0x3ad/0x870 [btrfs]
[ 3653.727739]        open_ctree+0xb8e/0x17bf [btrfs]
[ 3653.728384]        btrfs_mount_root.cold+0x12/0xde [btrfs]
[ 3653.729130]        legacy_get_tree+0x30/0x50
[ 3653.729676]        vfs_get_tree+0x28/0xc0
[ 3653.730192]        vfs_kern_mount.part.0+0x71/0xb0
[ 3653.730800]        btrfs_mount+0x11d/0x3a0 [btrfs]
[ 3653.731427]        legacy_get_tree+0x30/0x50
[ 3653.731970]        vfs_get_tree+0x28/0xc0
[ 3653.732486]        path_mount+0x2d4/0xbe0
[ 3653.732997]        __x64_sys_mount+0x103/0x140
[ 3653.733560]        do_syscall_64+0x3b/0xc0
[ 3653.734080]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 3653.734782]
               other info that might help us debug this:

[ 3653.735784] Chain exists of:
                 &disk->open_mutex --> sb_internal#2 --> btrfs-chunk-00

[ 3653.737123]  Possible unsafe locking scenario:

[ 3653.737865]        CPU0                    CPU1
[ 3653.738435]        ----                    ----
[ 3653.739007]   lock(btrfs-chunk-00);
[ 3653.739449]                                lock(sb_internal#2);
[ 3653.740193]                                lock(btrfs-chunk-00);
[ 3653.740955]   lock(&disk->open_mutex);
[ 3653.741431]
                *** DEADLOCK ***

[ 3653.742176] 3 locks held by mount/447465:
[ 3653.742739]  #0: ffff8c6acf85c0e8 (&type->s_umount_key#44/1){+.+.}-{3:3}, at: alloc_super+0xd5/0x3b0
[ 3653.744114]  #1: ffffffffc0b28f70 (uuid_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x59/0x870 [btrfs]
[ 3653.745563]  #2: ffff8c6b0a9f39e0 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x110 [btrfs]
[ 3653.747066]
               stack backtrace:
[ 3653.747723] CPU: 4 PID: 447465 Comm: mount Not tainted 5.15.0-rc7-btrfs-next-103 #1
[ 3653.748873] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 3653.750592] Call Trace:
[ 3653.750967]  dump_stack_lvl+0x57/0x72
[ 3653.751526]  check_noncircular+0xf3/0x110
[ 3653.752136]  ? stack_trace_save+0x4b/0x70
[ 3653.752748]  __lock_acquire+0x130e/0x2210
[ 3653.753356]  lock_acquire+0xd7/0x310
[ 3653.753898]  ? blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.754596]  ? lock_is_held_type+0xe8/0x140
[ 3653.755125]  ? blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.755729]  ? blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.756338]  __mutex_lock+0x92/0x900
[ 3653.756794]  ? blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.757400]  ? do_raw_spin_unlock+0x4b/0xa0
[ 3653.757930]  ? _raw_spin_unlock+0x29/0x40
[ 3653.758437]  ? bd_prepare_to_claim+0x129/0x150
[ 3653.758999]  ? trace_module_get+0x2b/0xd0
[ 3653.759508]  ? try_module_get.part.0+0x50/0x80
[ 3653.760072]  blkdev_get_by_dev.part.0+0xe7/0x320
[ 3653.760661]  ? devcgroup_check_permission+0xc1/0x1f0
[ 3653.761288]  blkdev_get_by_path+0xb8/0xd0
[ 3653.761797]  btrfs_get_bdev_and_sb+0x1b/0xb0 [btrfs]
[ 3653.762454]  open_fs_devices+0xd7/0x2c0 [btrfs]
[ 3653.763055]  ? clone_fs_devices+0x8f/0x170 [btrfs]
[ 3653.763689]  btrfs_read_chunk_tree+0x3ad/0x870 [btrfs]
[ 3653.764370]  ? kvm_sched_clock_read+0x14/0x40
[ 3653.764922]  open_ctree+0xb8e/0x17bf [btrfs]
[ 3653.765493]  ? super_setup_bdi_name+0x79/0xd0
[ 3653.766043]  btrfs_mount_root.cold+0x12/0xde [btrfs]
[ 3653.766780]  ? rcu_read_lock_sched_held+0x3f/0x80
[ 3653.767488]  ? kfree+0x1f2/0x3c0
[ 3653.767979]  legacy_get_tree+0x30/0x50
[ 3653.768548]  vfs_get_tree+0x28/0xc0
[ 3653.769076]  vfs_kern_mount.part.0+0x71/0xb0
[ 3653.769718]  btrfs_mount+0x11d/0x3a0 [btrfs]
[ 3653.770381]  ? rcu_read_lock_sched_held+0x3f/0x80
[ 3653.771086]  ? kfree+0x1f2/0x3c0
[ 3653.771574]  legacy_get_tree+0x30/0x50
[ 3653.772136]  vfs_get_tree+0x28/0xc0
[ 3653.772673]  path_mount+0x2d4/0xbe0
[ 3653.773201]  __x64_sys_mount+0x103/0x140
[ 3653.773793]  do_syscall_64+0x3b/0xc0
[ 3653.774333]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 3653.775094] RIP: 0033:0x7f648bc45aaa

This happens because through btrfs_read_chunk_tree(), which is called only
during mount, ends up acquiring the mutex open_mutex of a block device
while holding a read lock on a leaf of the chunk tree while other paths
need to acquire other locks before locking extent buffers of the chunk
tree.

Since at mount time when we call btrfs_read_chunk_tree() we know that
we don't have other tasks running in parallel and modifying the chunk
tree, we can simply skip locking of chunk tree extent buffers. So do
that and move the assertion that checks the fs is not yet mounted to the
top block of btrfs_read_chunk_tree(), with a comment before doing it.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>