Re: [PATCH v2] btrfs: fix lockdep warning while mounting sprout fs

[Anand Jain <anand.jain@oracle.com>]

Ping again.

Thanks, Anand

On 06/03/2021 16:37, Anand Jain wrote:
> 
> David,
> 
> Ping?
> 
> Thanks, Anand
> 
> 
> On 04/03/2021 02:10, Anand Jain wrote:
>> Following test case reproduces lockdep warning.
>>
>> Test case:
>> DEV1=/dev/vdb
>> DEV2=/dev/vdc
>> umount /btrfs
>> run mkfs.btrfs -f $DEV1
>> run btrfstune -S 1 $DEV1
>> run mount $DEV1 /btrfs
>> run btrfs device add $DEV2 /btrfs -f
>> run umount /btrfs
>> run mount $DEV2 /btrfs
>> run umount /btrfs
>>
>> 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
>>
>> [ 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
>>
>> [ 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().
>>
>> Reported-by: Su Yue <l@damenly.su>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>> 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 bc3b33efddc5..4188edbad2ef 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -570,6 +570,8 @@ 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;
>> @@ -1000,11 +1002,12 @@ 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) {
>> @@ -1036,10 +1039,8 @@ 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);
>>   }
>>
>