Re: [PATCH 2/2] mm/slub: Fix slab_mutex circular locking problem in slab_attr_store()

From: Waiman Long
Date: Mon Apr 27 2020 - 15:00:24 EST

On 4/24/20 3:44 PM, Qian Cai wrote:

On Apr 24, 2020, at 11:12 AM, Waiman Long <longman@xxxxxxxxxx> wrote:

The following lockdep splat was reported:

[ 176.241923] ======================================================
[ 176.241924] WARNING: possible circular locking dependency detected
[ 176.241926] 4.18.0-172.rt13.29.el8.x86_64+debug #1 Not tainted
[ 176.241927] ------------------------------------------------------
[ 176.241929] slub_cpu_partia/5371 is trying to acquire lock:
[ 176.241930] ffffffffa0b83718 (slab_mutex){+.+.}, at: slab_attr_store+0x6b/0xe0
[ 176.241941]
but task is already holding lock:
[ 176.241942] ffff88bb6d8b83c8 (kn->count#103){++++}, at: kernfs_fop_write+0x1cc/0x400
[ 176.241947]
which lock already depends on the new lock.

[ 176.241949]
the existing dependency chain (in reverse order) is:
[ 176.241949]
-> #1 (kn->count#103){++++}:
[ 176.241955] __kernfs_remove+0x616/0x800
[ 176.241957] kernfs_remove_by_name_ns+0x3e/0x80
[ 176.241959] sysfs_slab_add+0x1c6/0x330
[ 176.241961] __kmem_cache_create+0x15f/0x1b0
[ 176.241964] create_cache+0xe1/0x220
[ 176.241966] kmem_cache_create_usercopy+0x1a3/0x260
[ 176.241967] kmem_cache_create+0x12/0x20
[ 176.242076] mlx5_init_fs+0x18d/0x1a00 [mlx5_core]
[ 176.242100] mlx5_load_one+0x3b4/0x1730 [mlx5_core]
[ 176.242124] init_one+0x901/0x11b0 [mlx5_core]
[ 176.242127] local_pci_probe+0xd4/0x180
[ 176.242131] work_for_cpu_fn+0x51/0xa0
[ 176.242133] process_one_work+0x91a/0x1ac0
[ 176.242134] worker_thread+0x536/0xb40
[ 176.242136] kthread+0x30c/0x3d0
[ 176.242140] ret_from_fork+0x27/0x50
[ 176.242140]
-> #0 (slab_mutex){+.+.}:
[ 176.242145] __lock_acquire+0x22cb/0x48c0
[ 176.242146] lock_acquire+0x134/0x4c0
[ 176.242148] _mutex_lock+0x28/0x40
[ 176.242150] slab_attr_store+0x6b/0xe0
[ 176.242151] kernfs_fop_write+0x251/0x400
[ 176.242154] vfs_write+0x157/0x460
[ 176.242155] ksys_write+0xb8/0x170
[ 176.242158] do_syscall_64+0x13c/0x710
[ 176.242160] entry_SYSCALL_64_after_hwframe+0x6a/0xdf
[ 176.242161]
other info that might help us debug this:

[ 176.242161] Possible unsafe locking scenario:

[ 176.242162] CPU0 CPU1
[ 176.242163] ---- ----
[ 176.242163] lock(kn->count#103);
[ 176.242165] lock(slab_mutex);
[ 176.242166] lock(kn->count#103);
[ 176.242167] lock(slab_mutex);
[ 176.242169]
*** DEADLOCK ***

[ 176.242170] 3 locks held by slub_cpu_partia/5371:
[ 176.242170] #0: ffff888705e3a800 (sb_writers#4){.+.+}, at: vfs_write+0x31c/0x460
[ 176.242174] #1: ffff889aeec4d658 (&of->mutex){+.+.}, at: kernfs_fop_write+0x1a9/0x400
[ 176.242177] #2: ffff88bb6d8b83c8 (kn->count#103){++++}, at: kernfs_fop_write+0x1cc/0x400
[ 176.242180]
stack backtrace:
[ 176.242183] CPU: 36 PID: 5371 Comm: slub_cpu_partia Not tainted 4.18.0-172.rt13.29.el8.x86_64+debug #1
[ 176.242184] Hardware name: AMD Corporation DAYTONA_X/DAYTONA_X, BIOS RDY1005C 11/22/2019
[ 176.242185] Call Trace:
[ 176.242190] dump_stack+0x9a/0xf0
[ 176.242193] check_noncircular+0x317/0x3c0
[ 176.242195] ? print_circular_bug+0x1e0/0x1e0
[ 176.242199] ? native_sched_clock+0x32/0x1e0
[ 176.242202] ? sched_clock+0x5/0x10
[ 176.242205] ? sched_clock_cpu+0x238/0x340
[ 176.242208] __lock_acquire+0x22cb/0x48c0
[ 176.242213] ? trace_hardirqs_on+0x10/0x10
[ 176.242215] ? trace_hardirqs_on+0x10/0x10
[ 176.242218] lock_acquire+0x134/0x4c0
[ 176.242220] ? slab_attr_store+0x6b/0xe0
[ 176.242223] _mutex_lock+0x28/0x40
[ 176.242225] ? slab_attr_store+0x6b/0xe0
[ 176.242227] slab_attr_store+0x6b/0xe0
[ 176.242229] ? sysfs_file_ops+0x160/0x160
[ 176.242230] kernfs_fop_write+0x251/0x400
[ 176.242232] ? __sb_start_write+0x26a/0x3f0
[ 176.242234] vfs_write+0x157/0x460
[ 176.242237] ksys_write+0xb8/0x170
[ 176.242239] ? __ia32_sys_read+0xb0/0xb0
[ 176.242242] ? do_syscall_64+0xb9/0x710
[ 176.242245] do_syscall_64+0x13c/0x710
[ 176.242247] entry_SYSCALL_64_after_hwframe+0x6a/0xdf

There was another lockdep splat generated by echoing "1" to
"/sys/kernel/slab/fs_cache/shrink":

[ 445.231443] Chain exists of:
cpu_hotplug_lock --> mem_hotplug_lock --> slab_mutex

[ 445.242025] Possible unsafe locking scenario:

[ 445.247977] CPU0 CPU1
[ 445.252529] ---- ----
[ 445.257082] lock(slab_mutex);
[ 445.260239] lock(mem_hotplug_lock);
[ 445.266452] lock(slab_mutex);
[ 445.272141] lock(cpu_hotplug_lock);

So it is problematic to use slab_mutex to iterate the list of
child memcgs with for_each_memcg_cache(). Fortunately, there is
another way to do child memcg iteration by going through the array
entries in memcg_params.memcg_caches while holding a read lock on
memcg_cache_ids_sem.

To avoid other possible circular locking problems, we only take a
reference to the child memcgs and store their addresses while holding
memcg_cache_ids_sem. The actual store method is called for each of the
child memcgs after releasing the lock.
Even on x86 where it compiles, this patch is insufficient to prevent a lockdep splat,
because there are still cpu_hotplug_lock and mem_hotplug_lock in the way.

Yes, I was aware of that. Will include additional patch in v2 to address that.

Thanks,
Longman