Re: [PATCH V5 1/2] btrfs: fix lockdep warning while mounting sprout fs

[Nikolay Borisov <nborisov@suse.com>]

On 31.08.21 г. 4:21, Anand Jain wrote:
> Following test case reproduces lockdep warning.
> 
>  Test case:
> 
>  $ mkfs.btrfs -f <dev1>
>  $ btrfstune -S 1 <dev1>
>  $ mount <dev1> <mnt>
>  $ btrfs device add <dev2> <mnt> -f
>  $ umount <mnt>
>  $ mount <dev2> <mnt>
>  $ umount <mnt>
> 
> The warning claims a possible ABBA deadlock between the threads initiated by
> [#1] btrfs device add and [#0] the mount.
> 
> ======================================================
> [ 540.743122] WARNING: possible circular locking dependency detected
> [ 540.743129] 5.11.0-rc7+ #5 Not tainted
> [ 540.743135] ------------------------------------------------------
> [ 540.743142] mount/2515 is trying to acquire lock:
> [ 540.743149] ffffa0c5544c2ce0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: clone_fs_devices+0x6d/0x210 [btrfs]
> [ 540.743458] but task is already holding lock:
> [ 540.743461] ffffa0c54a7932b8 (btrfs-chunk-00){++++}-{4:4}, at: __btrfs_tree_read_lock+0x32/0x200 [btrfs]
> [ 540.743541] which lock already depends on the new lock.
> [ 540.743543] the existing dependency chain (in reverse order) is:
> 
> [ 540.743546] -> #1 (btrfs-chunk-00){++++}-{4:4}:
> [ 540.743566] down_read_nested+0x48/0x2b0
> [ 540.743585] __btrfs_tree_read_lock+0x32/0x200 [btrfs]
> [ 540.743650] btrfs_read_lock_root_node+0x70/0x200 [btrfs]
> [ 540.743733] btrfs_search_slot+0x6c6/0xe00 [btrfs]
> [ 540.743785] btrfs_update_device+0x83/0x260 [btrfs]
> [ 540.743849] btrfs_finish_chunk_alloc+0x13f/0x660 [btrfs] <--- device_list_mutex
> [ 540.743911] btrfs_create_pending_block_groups+0x18d/0x3f0 [btrfs]
> [ 540.743982] btrfs_commit_transaction+0x86/0x1260 [btrfs]
> [ 540.744037] btrfs_init_new_device+0x1600/0x1dd0 [btrfs]
> [ 540.744101] btrfs_ioctl+0x1c77/0x24c0 [btrfs]
> [ 540.744166] __x64_sys_ioctl+0xe4/0x140
> [ 540.744170] do_syscall_64+0x4b/0x80
> [ 540.744174] entry_SYSCALL_64_after_hwframe+0x44/0xa9

In this callchain we lock device_list_mutex followed by locking the
chunk root extent item
> 
> [ 540.744180] -> #0 (&fs_devs->device_list_mutex){+.+.}-{4:4}:
> [ 540.744184] __lock_acquire+0x155f/0x2360
> [ 540.744188] lock_acquire+0x10b/0x5c0
> [ 540.744190] __mutex_lock+0xb1/0xf80
> [ 540.744193] mutex_lock_nested+0x27/0x30
> [ 540.744196] clone_fs_devices+0x6d/0x210 [btrfs]
> [ 540.744270] btrfs_read_chunk_tree+0x3c7/0xbb0 [btrfs]
> [ 540.744336] open_ctree+0xf6e/0x2074 [btrfs]
> [ 540.744406] btrfs_mount_root.cold.72+0x16/0x127 [btrfs]
> [ 540.744472] legacy_get_tree+0x38/0x90
> [ 540.744475] vfs_get_tree+0x30/0x140
> [ 540.744478] fc_mount+0x16/0x60
> [ 540.744482] vfs_kern_mount+0x91/0x100
> [ 540.744484] btrfs_mount+0x1e6/0x670 [btrfs]
> [ 540.744536] legacy_get_tree+0x38/0x90
> [ 540.744537] vfs_get_tree+0x30/0x140
> [ 540.744539] path_mount+0x8d8/0x1070
> [ 540.744541] do_mount+0x8d/0xc0
> [ 540.744543] __x64_sys_mount+0x125/0x160
> [ 540.744545] do_syscall_64+0x4b/0x80
> [ 540.744547] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Whereas in this call chain we first lock the chunk_root in
btrfs_read_chunk_tree and then acquire device_list_mutex in
clone_fs_devices. And this inversion of lock order could potentially
lead to a deadlock.

> 
> [ 540.744551] other info that might help us debug this:
> [ 540.744552] Possible unsafe locking scenario:
> 
> [ 540.744553] CPU0 				CPU1
> [ 540.744554] ---- 				----
> [ 540.744555] lock(btrfs-chunk-00);
> [ 540.744557] 					lock(&fs_devs->device_list_mutex);
> [ 540.744560] 					lock(btrfs-chunk-00);
> [ 540.744562] lock(&fs_devs->device_list_mutex);
> [ 540.744564]
>  *** DEADLOCK ***
> 
> [ 540.744565] 3 locks held by mount/2515:
> [ 540.744567] #0: ffffa0c56bf7a0e0 (&type->s_umount_key#42/1){+.+.}-{4:4}, at: alloc_super.isra.16+0xdf/0x450
> [ 540.744574] #1: ffffffffc05a9628 (uuid_mutex){+.+.}-{4:4}, at: btrfs_read_chunk_tree+0x63/0xbb0 [btrfs]
> [ 540.744640] #2: ffffa0c54a7932b8 (btrfs-chunk-00){++++}-{4:4}, at: __btrfs_tree_read_lock+0x32/0x200 [btrfs]
> [ 540.744708]
>  stack backtrace:
> [ 540.744712] CPU: 2 PID: 2515 Comm: mount Not tainted 5.11.0-rc7+ #5
> 
> But the device_list_mutex in clone_fs_devices() is redundant, as explained
> below.
> Two threads [1]  and [2] (below) could lead to clone_fs_device().
> 
> [1]
> open_ctree <== mount sprout fs
>  btrfs_read_chunk_tree()
>   mutex_lock(&uuid_mutex) <== global lock
>   read_one_dev()
>    open_seed_devices()
>     clone_fs_devices() <== seed fs_devices
>      mutex_lock(&orig->device_list_mutex) <== seed fs_devices
> 
> [2]
> btrfs_init_new_device() <== sprouting
>  mutex_lock(&uuid_mutex); <== global lock
>  btrfs_prepare_sprout()
>    lockdep_assert_held(&uuid_mutex)
>    clone_fs_devices(seed_fs_device) <== seed fs_devices
> 
> Both of these threads hold uuid_mutex which is sufficient to protect
> getting the seed device(s) freed while we are trying to clone it for
> sprouting [2] or mounting a sprout [1] (as above). A mounted seed
> device can not free/write/replace because it is read-only. An unmounted
> seed device can free by btrfs_free_stale_devices(), but it needs uuid_mutex.
> So this patch removes the unnecessary device_list_mutex in clone_fs_devices().
> And adds a lockdep_assert_held(&uuid_mutex) in clone_fs_devices().

The reason we hold the device_list_mutex is not due to freeing (only)
but also due to possible racing mounts of filesystems seeded by the same
device. I.e in btrfs_prepare_sprout we essentially "empty" the seed
following the call to clone_fs_devices by splicing its devices on the
newly allocated 'seed_device'.

But since those processes are synchronized by uuid_mutex (i.e
open_seed_devices) then I'd say this is fine.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> 
> Reported-by: Su Yue <l@damenly.su>
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
> v5: Vet test case in the changelog.
> v2: Remove Martin's Reported-by and Tested-by.
>     Add Su's Reported-by.
>     Add lockdep_assert_held check.
>     Update the changelog, make it relevant to the current misc-next
> 
>  fs/btrfs/volumes.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index abdc392f6ef9..fa9fe47b5b68 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -558,6 +558,8 @@ static int btrfs_free_stale_devices(const char *path,
>  	struct btrfs_device *device, *tmp_device;
>  	int ret = 0;
>  
> +	lockdep_assert_held(&uuid_mutex);
> +
>  	if (path)
>  		ret = -ENOENT;
>  
> @@ -988,11 +990,12 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
>  	struct btrfs_device *orig_dev;
>  	int ret = 0;
>  
> +	lockdep_assert_held(&uuid_mutex);
> +
>  	fs_devices = alloc_fs_devices(orig->fsid, NULL);
>  	if (IS_ERR(fs_devices))
>  		return fs_devices;
>  
> -	mutex_lock(&orig->device_list_mutex);
>  	fs_devices->total_devices = orig->total_devices;
>  
>  	list_for_each_entry(orig_dev, &orig->devices, dev_list) {
> @@ -1024,10 +1027,8 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
>  		device->fs_devices = fs_devices;
>  		fs_devices->num_devices++;
>  	}
> -	mutex_unlock(&orig->device_list_mutex);
>  	return fs_devices;
>  error:
> -	mutex_unlock(&orig->device_list_mutex);
>  	free_fs_devices(fs_devices);
>  	return ERR_PTR(ret);
>  }
>