Re: [syzbot] [mm?] possible deadlock in rhashtable_free_and_destroy

Shakeel Butt
2026-04-21T21:27:54Z

Ccing relevant folks as this seems related to recent change of moving
simple_xattrs from rbtree to rhastable.

On Tue, Apr 21, 2026 at 08:34:22AM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 8541d8f725c6 Merge tag 'mtd/for-7.1' of git://git.kernel.o..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15380836580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=7e54da1916e8d11f
> dashboard link: https://syzkaller.appspot.com/bug?extid=5af806780f38a5fe691f
> compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-8541d8f7.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/22dfea2c37c2/vmlinux-8541d8f7.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/e2f93ad68fe3/bzImage-8541d8f7.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+5af806780f38a5fe691f@syzkaller.appspotmail.com
>
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Tainted: G L
> ------------------------------------------------------
> kswapd0/108 is trying to acquire lock:
> ffff888056f3c4e8 (&ht->mutex){+.+.}-{4:4}, at: rhashtable_free_and_destroy+0x3d/0x9b0 lib/rhashtable.c:1154
>
> but task is already holding lock:
> ffffffff8e9b0800 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat+0xb5d/0x1ac0 mm/vmscan.c:7102
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (fs_reclaim){+.+.}-{0:0}:
> __fs_reclaim_acquire mm/page_alloc.c:4327 [inline]
> fs_reclaim_acquire+0xc4/0x100 mm/page_alloc.c:4341
> might_alloc include/linux/sched/mm.h:317 [inline]
> slab_pre_alloc_hook mm/slub.c:4520 [inline]
> slab_alloc_node mm/slub.c:4875 [inline]
> __do_kmalloc_node mm/slub.c:5294 [inline]
> __kvmalloc_node_noprof+0xcc/0xa00 mm/slub.c:6828
> bucket_table_alloc.isra.0+0x88/0x460 lib/rhashtable.c:186
> rhashtable_rehash_alloc+0x68/0x110 lib/rhashtable.c:368
> rht_deferred_worker+0x1d9/0x1fd0 lib/rhashtable.c:429
> process_one_work+0xa0e/0x1980 kernel/workqueue.c:3302
> process_scheduled_works kernel/workqueue.c:3385 [inline]
> worker_thread+0x5ef/0xe50 kernel/workqueue.c:3466
> kthread+0x370/0x450 kernel/kthread.c:436
> ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> -> #0 (&ht->mutex){+.+.}-{4:4}:
> check_prev_add kernel/locking/lockdep.c:3165 [inline]
> check_prevs_add kernel/locking/lockdep.c:3284 [inline]
> validate_chain kernel/locking/lockdep.c:3908 [inline]
> __lock_acquire+0x14b8/0x2630 kernel/locking/lockdep.c:5237
> lock_acquire kernel/locking/lockdep.c:5868 [inline]
> lock_acquire+0x1b1/0x370 kernel/locking/lockdep.c:5825
> __mutex_lock_common kernel/locking/mutex.c:632 [inline]
> __mutex_lock+0x1a4/0x1b10 kernel/locking/mutex.c:806

The cancel_work_sync(&ht->run_work) in rhashtable_free_and_destroy() should
avoid concurrent mutex lockers from rht_deferred_worker(). Seems like false
positive as all rhastables share single static lockdep class for the ht->mutex.

Simple fix would be to move rhashtable_rehash_alloc() to use NOFS allocation or
we can introduce more fine grained lockdep classes for rhashtables.

> rhashtable_free_and_destroy+0x3d/0x9b0 lib/rhashtable.c:1154
> shmem_evict_inode+0x1ae/0xc40 mm/shmem.c:1429
> evict+0x3c2/0xad0 fs/inode.c:841
> iput_final fs/inode.c:1960 [inline]
> iput.part.0+0x605/0xf50 fs/inode.c:2009
> iput+0x35/0x40 fs/inode.c:1975
> dentry_unlink_inode+0x2a1/0x490 fs/dcache.c:467
> __dentry_kill+0x1d0/0x600 fs/dcache.c:670
> finish_dput+0x76/0x480 fs/dcache.c:879
> dput.part.0+0x456/0x570 fs/dcache.c:928
> dput+0x1f/0x30 fs/dcache.c:920
> ovl_destroy_inode+0x3e/0x190 fs/overlayfs/super.c:217
> destroy_inode+0xcb/0x1c0 fs/inode.c:394
> evict+0x599/0xad0 fs/inode.c:865
> iput_final fs/inode.c:1960 [inline]
> iput.part.0+0x605/0xf50 fs/inode.c:2009
> iput+0x35/0x40 fs/inode.c:1975
> dentry_unlink_inode+0x2a1/0x490 fs/dcache.c:467
> __dentry_kill+0x1d0/0x600 fs/dcache.c:670
> shrink_kill fs/dcache.c:1147 [inline]
> shrink_dentry_list+0x180/0x5e0 fs/dcache.c:1174
> prune_dcache_sb+0xea/0x150 fs/dcache.c:1256
> super_cache_scan+0x328/0x550 fs/super.c:223
> do_shrink_slab+0x416/0x1240 mm/shrinker.c:440
> shrink_slab_memcg mm/shrinker.c:557 [inline]
> shrink_slab+0xa7d/0x12e0 mm/shrinker.c:635
> shrink_one+0x398/0x7f0 mm/vmscan.c:4932
> shrink_many mm/vmscan.c:4993 [inline]
> lru_gen_shrink_node mm/vmscan.c:5071 [inline]
> shrink_node+0x2673/0x3dc0 mm/vmscan.c:6059
> kswapd_shrink_node mm/vmscan.c:6913 [inline]
> balance_pgdat+0xaaf/0x1ac0 mm/vmscan.c:7089
> kswapd+0x557/0xb60 mm/vmscan.c:7362
> kthread+0x370/0x450 kernel/kthread.c:436
> ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(fs_reclaim);
> lock(&ht->mutex);
> lock(fs_reclaim);
> lock(&ht->mutex);
>
> *** DEADLOCK ***
>
> 2 locks held by kswapd0/108:
> #0: ffffffff8e9b0800 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat+0xb5d/0x1ac0 mm/vmscan.c:7102
> #1: ffff88801347c0d8 (&type->s_umount_key#76){++++}-{4:4}, at: super_trylock_shared fs/super.c:565 [inline]
> #1: ffff88801347c0d8 (&type->s_umount_key#76){++++}-{4:4}, at: super_cache_scan+0x98/0x550 fs/super.c:198
>
> stack backtrace:
> CPU: 2 UID: 0 PID: 108 Comm: kswapd0 Tainted: G L syzkaller #0 PREEMPT(full)
> Tainted: [L]=SOFTLOCKUP
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:94 [inline]
> dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
> print_circular_bug.cold+0x178/0x1c7 kernel/locking/lockdep.c:2043
> check_noncircular+0x146/0x160 kernel/locking/lockdep.c:2175
> check_prev_add kernel/locking/lockdep.c:3165 [inline]
> check_prevs_add kernel/locking/lockdep.c:3284 [inline]
> validate_chain kernel/locking/lockdep.c:3908 [inline]
> __lock_acquire+0x14b8/0x2630 kernel/locking/lockdep.c:5237
> lock_acquire kernel/locking/lockdep.c:5868 [inline]
> lock_acquire+0x1b1/0x370 kernel/locking/lockdep.c:5825
> __mutex_lock_common kernel/locking/mutex.c:632 [inline]
> __mutex_lock+0x1a4/0x1b10 kernel/locking/mutex.c:806
> rhashtable_free_and_destroy+0x3d/0x9b0 lib/rhashtable.c:1154
> shmem_evict_inode+0x1ae/0xc40 mm/shmem.c:1429
> evict+0x3c2/0xad0 fs/inode.c:841
> iput_final fs/inode.c:1960 [inline]
> iput.part.0+0x605/0xf50 fs/inode.c:2009
> iput+0x35/0x40 fs/inode.c:1975
> dentry_unlink_inode+0x2a1/0x490 fs/dcache.c:467
> __dentry_kill+0x1d0/0x600 fs/dcache.c:670
> finish_dput+0x76/0x480 fs/dcache.c:879
> dput.part.0+0x456/0x570 fs/dcache.c:928
> dput+0x1f/0x30 fs/dcache.c:920
> ovl_destroy_inode+0x3e/0x190 fs/overlayfs/super.c:217
> destroy_inode+0xcb/0x1c0 fs/inode.c:394
> evict+0x599/0xad0 fs/inode.c:865
> iput_final fs/inode.c:1960 [inline]
> iput.part.0+0x605/0xf50 fs/inode.c:2009
> iput+0x35/0x40 fs/inode.c:1975
> dentry_unlink_inode+0x2a1/0x490 fs/dcache.c:467
> __dentry_kill+0x1d0/0x600 fs/dcache.c:670
> shrink_kill fs/dcache.c:1147 [inline]
> shrink_dentry_list+0x180/0x5e0 fs/dcache.c:1174
> prune_dcache_sb+0xea/0x150 fs/dcache.c:1256
> super_cache_scan+0x328/0x550 fs/super.c:223
> do_shrink_slab+0x416/0x1240 mm/shrinker.c:440
> shrink_slab_memcg mm/shrinker.c:557 [inline]
> shrink_slab+0xa7d/0x12e0 mm/shrinker.c:635
> shrink_one+0x398/0x7f0 mm/vmscan.c:4932
> shrink_many mm/vmscan.c:4993 [inline]
> lru_gen_shrink_node mm/vmscan.c:5071 [inline]
> shrink_node+0x2673/0x3dc0 mm/vmscan.c:6059
> kswapd_shrink_node mm/vmscan.c:6913 [inline]
> balance_pgdat+0xaaf/0x1ac0 mm/vmscan.c:7089
> kswapd+0x557/0xb60 mm/vmscan.c:7362
> kthread+0x370/0x450 kernel/kthread.c:436
> ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> </TASK>
>
>
> ---
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