Re: [PATCH] btrfs: fix lockdep splat in add_missing_dev

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

On 31/8/20 10:52 pm, Josef Bacik wrote:
> Nikolay reported a lockdep splat that I could reproduce with btrfs/187.
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.9.0-rc2+ #1 Tainted: G        W
> ------------------------------------------------------
> kswapd0/100 is trying to acquire lock:
> ffff9e8ef38b6268 (&delayed_node->mutex){+.+.}-{3:3}, at: __btrfs_release_delayed_node.part.0+0x3f/0x330
> 
> but task is already holding lock:
> ffffffffa9d74700 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #2 (fs_reclaim){+.+.}-{0:0}:
>         fs_reclaim_acquire+0x65/0x80
>         slab_pre_alloc_hook.constprop.0+0x20/0x200
>         kmem_cache_alloc_trace+0x3a/0x1a0
>         btrfs_alloc_device+0x43/0x210
>         add_missing_dev+0x20/0x90
>         read_one_chunk+0x301/0x430
>         btrfs_read_sys_array+0x17b/0x1b0
>         open_ctree+0xa62/0x1896
>         btrfs_mount_root.cold+0x12/0xea
>         legacy_get_tree+0x30/0x50
>         vfs_get_tree+0x28/0xc0
>         vfs_kern_mount.part.0+0x71/0xb0
>         btrfs_mount+0x10d/0x379
>         legacy_get_tree+0x30/0x50
>         vfs_get_tree+0x28/0xc0
>         path_mount+0x434/0xc00
>         __x64_sys_mount+0xe3/0x120
>         do_syscall_64+0x33/0x40
>         entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> -> #1 (&fs_info->chunk_mutex){+.+.}-{3:3}:
>         __mutex_lock+0x7e/0x7e0
>         btrfs_chunk_alloc+0x125/0x3a0
>         find_free_extent+0xdf6/0x1210
>         btrfs_reserve_extent+0xb3/0x1b0
>         btrfs_alloc_tree_block+0xb0/0x310
>         alloc_tree_block_no_bg_flush+0x4a/0x60
>         __btrfs_cow_block+0x11a/0x530
>         btrfs_cow_block+0x104/0x220
>         btrfs_search_slot+0x52e/0x9d0
>         btrfs_lookup_inode+0x2a/0x8f
>         __btrfs_update_delayed_inode+0x80/0x240
>         btrfs_commit_inode_delayed_inode+0x119/0x120
>         btrfs_evict_inode+0x357/0x500
>         evict+0xcf/0x1f0
>         vfs_rmdir.part.0+0x149/0x160
>         do_rmdir+0x136/0x1a0
>         do_syscall_64+0x33/0x40
>         entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> -> #0 (&delayed_node->mutex){+.+.}-{3:3}:
>         __lock_acquire+0x1184/0x1fa0
>         lock_acquire+0xa4/0x3d0
>         __mutex_lock+0x7e/0x7e0
>         __btrfs_release_delayed_node.part.0+0x3f/0x330
>         btrfs_evict_inode+0x24c/0x500
>         evict+0xcf/0x1f0
>         dispose_list+0x48/0x70
>         prune_icache_sb+0x44/0x50
>         super_cache_scan+0x161/0x1e0
>         do_shrink_slab+0x178/0x3c0
>         shrink_slab+0x17c/0x290
>         shrink_node+0x2b2/0x6d0
>         balance_pgdat+0x30a/0x670
>         kswapd+0x213/0x4c0
>         kthread+0x138/0x160
>         ret_from_fork+0x1f/0x30
> 
> other info that might help us debug this:
> 
> Chain exists of:
>    &delayed_node->mutex --> &fs_info->chunk_mutex --> fs_reclaim
> 
>   Possible unsafe locking scenario:
> 
>         CPU0                    CPU1
>         ----                    ----
>    lock(fs_reclaim);
>                                 lock(&fs_info->chunk_mutex);
>                                 lock(fs_reclaim);
>    lock(&delayed_node->mutex);
> 
>   *** DEADLOCK ***
> 
> 3 locks held by kswapd0/100:
>   #0: ffffffffa9d74700 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30
>   #1: ffffffffa9d65c50 (shrinker_rwsem){++++}-{3:3}, at: shrink_slab+0x115/0x290
>   #2: ffff9e8e9da260e0 (&type->s_umount_key#48){++++}-{3:3}, at: super_cache_scan+0x38/0x1e0
> 
> stack backtrace:
> CPU: 1 PID: 100 Comm: kswapd0 Tainted: G        W         5.9.0-rc2+ #1
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
> Call Trace:
>   dump_stack+0x92/0xc8
>   check_noncircular+0x12d/0x150
>   __lock_acquire+0x1184/0x1fa0
>   lock_acquire+0xa4/0x3d0
>   ? __btrfs_release_delayed_node.part.0+0x3f/0x330
>   __mutex_lock+0x7e/0x7e0
>   ? __btrfs_release_delayed_node.part.0+0x3f/0x330
>   ? __btrfs_release_delayed_node.part.0+0x3f/0x330
>   ? lock_acquire+0xa4/0x3d0
>   ? btrfs_evict_inode+0x11e/0x500
>   ? find_held_lock+0x2b/0x80
>   __btrfs_release_delayed_node.part.0+0x3f/0x330
>   btrfs_evict_inode+0x24c/0x500
>   evict+0xcf/0x1f0
>   dispose_list+0x48/0x70
>   prune_icache_sb+0x44/0x50
>   super_cache_scan+0x161/0x1e0
>   do_shrink_slab+0x178/0x3c0
>   shrink_slab+0x17c/0x290
>   shrink_node+0x2b2/0x6d0
>   balance_pgdat+0x30a/0x670
>   kswapd+0x213/0x4c0
>   ? _raw_spin_unlock_irqrestore+0x46/0x60
>   ? add_wait_queue_exclusive+0x70/0x70
>   ? balance_pgdat+0x670/0x670
>   kthread+0x138/0x160
>   ? kthread_create_worker_on_cpu+0x40/0x40
>   ret_from_fork+0x1f/0x30
> 
> This is because we are holding the chunk_mutex when we call
> btrfs_alloc_device, which does a GFP_KERNEL allocation.  We don't want
> to switch that to a GFP_NOFS lock because this is the only place where
> it matters.  So instead use memalloc_nofs_save() around the allocation
> in order to avoid the lockdep splat.
> 
> References: https://github.com/btrfs/fstests/issues/6
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>


Reviewed-by: Anand Jain <anand.jain@oracle.com>


Thanks. Anand