Re: [PATCH] btrfs: nofs inode allocations

Nikolay Borisov
2019-09-09T14:28:16Z

On 9.09.19 г. 17:12 ч., Josef Bacik wrote:
> A user reported a lockdep splat
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.2.11-gentoo #2 Not tainted
> ------------------------------------------------------
> kswapd0/711 is trying to acquire lock:
> 000000007777a663 (sb_internal){.+.+}, at: start_transaction+0x3a8/0x500
>
> but task is already holding lock:
> 000000000ba86300 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x30
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (fs_reclaim){+.+.}:
> kmem_cache_alloc+0x1f/0x1c0
> btrfs_alloc_inode+0x1f/0x260
> alloc_inode+0x16/0xa0
> new_inode+0xe/0xb0
> btrfs_new_inode+0x70/0x610
> btrfs_symlink+0xd0/0x420
> vfs_symlink+0x9c/0x100
> do_symlinkat+0x66/0xe0
> do_syscall_64+0x55/0x1c0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> #0 (sb_internal){.+.+}:
> __sb_start_write+0xf6/0x150
> start_transaction+0x3a8/0x500
> btrfs_commit_inode_delayed_inode+0x59/0x110
> btrfs_evict_inode+0x19e/0x4c0
> evict+0xbc/0x1f0
> inode_lru_isolate+0x113/0x190
> __list_lru_walk_one.isra.4+0x5c/0x100
> list_lru_walk_one+0x32/0x50
> prune_icache_sb+0x36/0x80
> super_cache_scan+0x14a/0x1d0
> do_shrink_slab+0x131/0x320
> shrink_node+0xf7/0x380
> balance_pgdat+0x2d5/0x640
> kswapd+0x2ba/0x5e0
> kthread+0x147/0x160
> ret_from_fork+0x24/0x30
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(fs_reclaim);
> lock(sb_internal);
> lock(fs_reclaim);
> lock(sb_internal);
> *** DEADLOCK ***
>
> 3 locks held by kswapd0/711:
> #0: 000000000ba86300 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x30
> #1: 000000004a5100f8 (shrinker_rwsem){++++}, at: shrink_node+0x9a/0x380
> #2: 00000000f956fa46 (&type->s_umount_key#30){++++}, at: super_cache_scan+0x35/0x1d0
>
> stack backtrace:
> CPU: 7 PID: 711 Comm: kswapd0 Not tainted 5.2.11-gentoo #2
> Hardware name: Dell Inc. Precision Tower 3620/0MWYPT, BIOS 2.4.2 09/29/2017
> Call Trace:
> dump_stack+0x85/0xc7
> print_circular_bug.cold.40+0x1d9/0x235
> __lock_acquire+0x18b1/0x1f00
> lock_acquire+0xa6/0x170
> ? start_transaction+0x3a8/0x500
> __sb_start_write+0xf6/0x150
> ? start_transaction+0x3a8/0x500
> start_transaction+0x3a8/0x500
> btrfs_commit_inode_delayed_inode+0x59/0x110
> btrfs_evict_inode+0x19e/0x4c0
> ? var_wake_function+0x20/0x20
> evict+0xbc/0x1f0
> inode_lru_isolate+0x113/0x190
> ? discard_new_inode+0xc0/0xc0
> __list_lru_walk_one.isra.4+0x5c/0x100
> ? discard_new_inode+0xc0/0xc0
> list_lru_walk_one+0x32/0x50
> prune_icache_sb+0x36/0x80
> super_cache_scan+0x14a/0x1d0
> do_shrink_slab+0x131/0x320
> shrink_node+0xf7/0x380
> balance_pgdat+0x2d5/0x640
> kswapd+0x2ba/0x5e0
> ? __wake_up_common_lock+0x90/0x90
> kthread+0x147/0x160
> ? balance_pgdat+0x640/0x640
> ? __kthread_create_on_node+0x160/0x160
> ret_from_fork+0x24/0x30
>
> This is because btrfs_new_inode() calls new_inode() under the
> transaction. We could probably move the new_inode() outside of this but
> for now just wrap it in memalloc_nofs_save().

If I'm understanding correctly what happens here is that symlinking
wants to instantiate a new inode
(new_inode->btrfs_alloc_inode->kmem_cache_alloc with GFP_KERNEL) but it
triggers background reclaim, hence goes to sleep, while holding a
transaction. At the same time background reclaim joins the same
transaction which is now blocked by the symlinking thread, hence it's
prone to deadlock ?