Re: [stable] ext4 fscrypt_get_encryption_info() circular locking dependency

Eric Biggers
2020-12-11T03:50:00Z

On Fri, Dec 11, 2020 at 12:36:57PM +0900, Sergey Senozhatsky wrote:
> Hi,
>
> I got the following lockdep splat the other day, while running some
> tests on 4.19. I didn't test other stable kernels, but it seems that
> 5.4 should also have similar problem.
>
> As far as I can tell, ext4_dir_open() has been removed quite recently:
> https://www.mail-archive.com/linux-f2fs-devel@lists.sourceforge.net/msg18727.html
>
> Eric, Ted, Jaegeuk, how difficult would it be to remove ext4_dir_open()
> from the stable kernels? (I'm interested in ext4 in particular, I
> understand that other filesystems may also need similar patches)
>
>
>
> [ 133.454721] kswapd0/79 is trying to acquire lock:
> [ 133.454724] 00000000a815a55f (jbd2_handle){++++}, at: start_this_handle+0x1f9/0x859
> [ 133.454730]
> but task is already holding lock:
> [ 133.454731] 00000000106bd5a3 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x5/0x2f
> [ 133.454736]
> which lock already depends on the new lock.
>
> [ 133.454739]
> the existing dependency chain (in reverse order) is:
> [ 133.454740]
> -> #2 (fs_reclaim){+.+.}:
> [ 133.454745] kmem_cache_alloc_trace+0x44/0x28b
> [ 133.454748] mempool_create_node+0x46/0x92
> [ 133.454750] fscrypt_initialize+0xa0/0xbf
> [ 133.454752] fscrypt_get_encryption_info+0xa4/0x774
> [ 133.454754] ext4_dir_open+0x1b/0x2d
> [ 133.454757] do_dentry_open+0x144/0x36d
> [ 133.454759] path_openat+0x2d7/0x156d
> [ 133.454762] do_filp_open+0x97/0x13e
> [ 133.454764] do_sys_open+0x128/0x3a3
> [ 133.454766] do_syscall_64+0x6f/0x22a
> [ 133.454769] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> [ 133.454771]
> -> #1 (fscrypt_init_mutex){+.+.}:
> [ 133.454774] mutex_lock_nested+0x20/0x26
> [ 133.454776] fscrypt_initialize+0x20/0xbf
> [ 133.454778] fscrypt_get_encryption_info+0xa4/0x774
> [ 133.454780] fscrypt_inherit_context+0xbe/0xe6
> [ 133.454782] __ext4_new_inode+0x11ee/0x1631
> [ 133.454785] ext4_mkdir+0x112/0x416
> [ 133.454787] vfs_mkdir2+0x135/0x1c6
> [ 133.454789] do_mkdirat+0xc3/0x138
> [ 133.454791] do_syscall_64+0x6f/0x22a
> [ 133.454793] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> [ 133.454795]
> -> #0 (jbd2_handle){++++}:
> [ 133.454798] start_this_handle+0x21c/0x859
> [ 133.454800] jbd2__journal_start+0xa2/0x282
> [ 133.454802] ext4_release_dquot+0x58/0x93
> [ 133.454804] dqput+0x196/0x1ec
> [ 133.454806] __dquot_drop+0x8d/0xb2
> [ 133.454809] ext4_clear_inode+0x22/0x8c
> [ 133.454811] ext4_evict_inode+0x127/0x662
> [ 133.454813] evict+0xc0/0x241
> [ 133.454816] dispose_list+0x36/0x54
> [ 133.454818] prune_icache_sb+0x56/0x76
> [ 133.454820] super_cache_scan+0x13a/0x19c
> [ 133.454822] shrink_slab+0x39a/0x572
> [ 133.454824] shrink_node+0x3f8/0x63b
> [ 133.454826] balance_pgdat+0x1bd/0x326
> [ 133.454828] kswapd+0x2ad/0x510
> [ 133.454831] kthread+0x14d/0x155
> [ 133.454833] ret_from_fork+0x24/0x50
> [ 133.454834]
> other info that might help us debug this:
>
> [ 133.454836] Chain exists of:
> jbd2_handle --> fscrypt_init_mutex --> fs_reclaim
>
> [ 133.454840] Possible unsafe locking scenario:
>
> [ 133.454841] CPU0 CPU1
> [ 133.454843] ---- ----
> [ 133.454844] lock(fs_reclaim);
> [ 133.454846] lock(fscrypt_init_mutex);
> [ 133.454848] lock(fs_reclaim);
> [ 133.454850] lock(jbd2_handle);
> [ 133.454851]

This actually got fixed by the patch series
https://lkml.kernel.org/linux-fscrypt/20200913083620.170627-1-ebiggers@kernel.org/
which went into 5.10. The more recent patch to remove ext4_dir_open() isn't
related.

It's a hard patch series to backport. Backporting it to 5.4 would be somewhat
feasible, while 4.19 would be very difficult as there have been a lot of other
fscrypt commits which would heavily conflict with cherry-picks.

How interested are you in having this fixed? Did you encounter an actual
deadlock or just the lockdep report?

- Eric